Since it's been such a long time since I've done any kind of written rants, and tomorrow I'll be posting an LP video that focuses exclusively on the Join Avenue, I think it's an appropriate time to share my thoughts on each of the shops that can be found there. First, here's the reference you should be using whenever you want to look up something. It withholds a few details that I know of, though, so I'll point these out when I can.
Unfortunately there's no list of odds at each rank that I can find, so all we know is that the odds of getting better prizes are increased as you level up the raffle shop. Oddly enough, the Berry Juice, the "failure" item and the least useful on paper is more desirable than the Full Heal or the Ultra Ball, as the maniac on route 5 will give you $1500 for it, more money than either of these items is worth!
All the other items on the list can't be bought anywhere, aside from PP Ups at a level 9 or 10 Market and Max Revives at a level 10 Market (and that obviously requires some effort, AND you can only buy one per day anyway). The Rare Candy also has a few counterparts in the Join Avenue that are much easier to access, though you need to fork over some dough for these. As far as the PP Up and PP Max go, you can still get a Star Piece a day from the challenge rock on the outskirts of Pinwheel Forest, though it's post-game only now, and trade it at Anville Town on weekends.
This means that although you won't say no to a free copy of any of these items, the only things you can really only get here, aside from the coveted Master Ball, are Max Ethers and Max Elixirs. Of course you can buy Ethers and Leppa Berries at other shops in the avenue, but Max Ethers and Max Elixirs are off-limits outside of the Raffle Shop.
So should you level it up? I guess it all depends on the appeal the items, and the fact that they're free, have for you. In the long term you should aim to max out all your shops anyway, but there are others that are worth focusing on more than this one.
First, what Bulbagarden doesn't tell you: Brushing, Shampoo, Smile Cut, Beauty Cut and Royal Cut increase happiness by 10, 20, 30, 40 and 50 respectively. This means that the more expensive ones give you more bang for your buck. Sadly, this means those who end up with the A variant get a bit gypped, but it's nothing to lose sleep over.
With that out of the way, just by looking at the available products you can see that this place is all about lowering EVs, so if you don't plan on lowering anything's EVs (or have a huge stock of EV-reducing berries in the Dream World) you can just level it up to 2 and be done with it (unless you meet an NPC who won't come anywhere but here, of course). Also note that you can buy EV-reducing berries in a level 9 or 10 Flower Shop, and they're a slightly better deal too. Problem is that you can only buy one package of four of each kind of berry per day, and only three of these are available to you no matter what (unless you make more than one flower shop, which I don't recommend), so the Beauty Salon isn't completely obsoleted. Especially considering the price difference isn't very significant to start with, and leveling up the latter to 7 gives you access to EV reduction for all stats right then and there, while leveling up a Flower Shop all the way still only gives you three kinds of EV reduction depending on the variant you get.
All about EV training... and raising levels. Believe it or not, as soon as you open a Dojo you have the option to raise a Pokémon by one level for $15000, and you can do it once every day! Increasing the Dojo's rank will give you even more possibilities, and once it's at level 10 you can potentially gain a total of TEN levels in a single day! Of course it's on the expensive side, which is where building up the Avenue's rank comes in useful - at rank 41 and above, all prices are cut by 40%, making the Dojo a prime stop for leveling up if you have some money to spend. As if it wasn't enough, all variants have all the leveling-up services, so there's no getting screwed over there!
The rest of the services are all for boosting EVs. In terms of cost efficiency, they lose out to vitamins - except for two important caveats. First, there's the aforementioned discount, which makes them slightly more cost-effective when your Avenue rank starts hitting the late 30s (...except when you buy said vitamins from the Market), AND they also work when your EV count for that particular stat is above 100, which isn't the case for vitamins. Of course, there's the whole once-per-day problem that rears its ugly head, but it can still make things significantly easier for you if you're willing to shell out the cash.
Regardless, if you want a quick way to level up your high-leveled Pokémon when Audinos, the Nimbasa stadiums and Black Tower / White Treehollow don't cut it anymore, don't hesitate and max this sucker out as soon as you can!
Another favorite of mine, the Market provides you with an opportunity to buy items in bulk at a discount... even when you're just starting your avenue out! Indeed, aside from Fresh Water and Moo Moo Milk, buying any set of items will be much cheaper than doing the same at a regular old vendor. So when you start building up that rank-based discount, just imagine how cheap these items will get!
Unfortunately, some of the more high-end items are only available later (and by that I mean Avenue rank 15 and E4 beaten), but it's not the end of the world, either. After all, when you DO gain access to them, Hyper Potions, not Moo Moo Milks and Lemonades, become the most cost-effective healing items in the game. Max Repels also get the same boost compared to Super Repels, and Revives... well, they're Revives, what else can I say?
As far as non-healing items go, you can also get some great stuff. Although some of these items are jokes, such as Honey and the overpriced Ragecandybars, Lava Cookies and Old Gateaux, you can buy X items and vitamins for much cheaper than you could elsewhere (in fact, vitamins bought here are more cost-effective than EV training at the Dojo), and you can get the not-buyable-anywhere-else Ethers, PP Ups and Max Revives at ranks 4, 9 and 10 respectively. These are very much worth maxing the Market out for, so definitely work on that as much as you can!
Like the Raffle Shop and the Salon, whether you want to level this up entirely depends on what you're looking for. No matter what, I suggest leveling it up to at least 3. Aside from the Pecha Berry, all the other status-healing berries, the Lum Berry even moreso, are much cheaper than the corresponding healing items. Of course, the Oran and Sitrus berries are the odd ones out, as they're rarely ever useful unless you attach them to your Pokémon (in fact Orans are completely obsolete by the point you reach the Join Avenue for the first time), and I much prefer giving mine non-consumables with a more consistent effect.
Rank 4 has the Leppa Berry, which is completely identical to the Ether unlocked by the rank 4 Market, same price and everything, except for the auto-consume-when-attached effect of course. If you feel you need both an Ether and a Leppa Berry every single day (though I don't see why it'd be so important to have both), do feel free to level up your Flower Shop to 4. As a side note, you can buy Razz or Bluk Berries from a rank 4 or above Flower Shop with the B or C variant, then sell them to the maniac on route 5 for a profit. (The Mulches you can buy here can also be sold to a maniac in Mistralton for a similar result - in fact that's their only effect this generation).
The rest of the berries up to rank 8 are weakness-dampening and pinch berries, which do have some interest in competitive play... too bad I don't advocate doing so on actual cartridges anyway. Nonetheless, if you elect to do so regardless, you might want to give the Flower Shop more love than a strictly in-game player would.
As for berries unlocked by ranks 9 and 10, they are the EV-reducing berries, which as I said before are more cost-effective than the Beauty Salon. However, as I mentioned before, the price difference is barely noticeable, AND you only get three of the six berries depending on your variant, whereas just leveling up a Beauty Salon to 7 gives you potential EV reduction for every stat.
Bottom line, it's entirely up to you how far you want to go with your Flower Shop, though for most purposes anything higher than rank 4 is overkill.
Once again, sadly I can't seem to find the odds anywhere for any of the items offered here. Like the Raffle Shop, you get a random prize from a pool of possible items, though this time you can get rare stuff like Nuggets, evolutionary items and fossils. Unlike the Raffle Shop however, you have to pay a small fee to participate, and leveling up the shop gives you access to better drawings (which are in turn more expensive) instead of improving the existing ones. A side effect of this is that you can get multiple items in one day from the Antique Shop.
Overall, how much you want to level it up depends on how much you want the items you can win here. Fossils are kind of pointless, so I suggest you don't even bother with the "Rock" items, but the others can have interesting stuff. It's worth noting that Funfest Missions are the best way to get shards, so the "Shard" items are really only interesting for the prospect of Nuggets.
As far as I'm concerned, the main reason why you'd level this up is if you intend on filling up your Pokédex - then you're going to need a lot of evolution items, and this could be a big help. Otherwise... I'd say send your NPCs in more useful shops if you can.
Quite honestly, I don't know why this exists. Friendship is taken care of by the Salon, and EVs and levels by the Dojo. The Friendly Drink, Lunch and Combo increase happiness by 5, 10 and 20 respectively, for an inferior cost effectiveness to that of the Salon. Same for Power Lunches compared to the Dojo, though the Secret Dishes match it.
The main reason why you'd want to level up this place, honestly, is if the levels provided by the Dojo aren't enough. The Wonder Brunch and Wonder Dessert feature the same $15000/level pattern as the Dojo, but the Loyalty Meal gives you one level for slightly cheaper, and the grand prize is the Wonder Dinner, which gives EIGHT levels for $96000 (that's $12000 per level if you do the math). Does it make leveling up a Café worth it? It depends on how badly you need these extra levels on top of what the Dojo already provides. And quite frankly, ten levels per day is already a sweet deal as it is.
Do you hatch a lot of eggs? If so, then this will help you slash significantly into the time you spend on Skyarrow Bridge hatching them... at higher levels. Warm level 1 only reduces the time needed to hatch an egg by 5%... even at $600 for three uses, that's kind of lame. Level 2 (available at rank 2) increases that to 30%, which is more interesting... but it's at level 3 (available at rank 4) that it really starts being worth using. Instead of reducing hatching time by a percentage, it reduces it by the fixed amount of 5120 steps... which halves the hatching time of even those that take the longest to hatch, and makes many of the others into an almost instantaneous affair. Levels 4 and 5 (available at ranks 8 and 10 respectively) increase that amount to 7680 and 10240, which makes their convenience unparalleled.
So, do you want to level that up? Rank 10 gives you six uses of Warm level 5 for just $2500, so if you're into breeding it will definitely prove to be very useful. Otherwise... it's not really worth bothering with it, though leveling it up to 4 to get access to Warm level 3 will be convenient if you're an occasional breeder.
One important thing to note is that the Nursery only becomes available after beating the E4 (makes sense, since whether you like it or not, breeding is a post-game only affair in this game) and reaching Avenue rank 15. You'd be surprised how many people don't even know it exists because of that.