First, Mega Glalie remains pure Ice, but has Refrigerate, which... well, it's a step up from what it previously had, if nothing else, especially with the lack of Icicle Crash. However, it now has the strongest detonation in the game. Will it be useful in any way? Maybe not, because its stat spread is 80/120/80/120/80/100, which means a lot of points wasted in special attack. Too bad. But, it gets something nothing else gets: the potential for super-effective Explosion, which no one else can boast. If anything at all can redeem Mega Glalie, it'll be this.
Mega Steelix looks more promising, with stats of 75/125/230/55/95/30, which is just about ideal, really. A bit less in defense and a bit more in attack or special defense would've been great, but it's still much better off than Mega Glalie. Shame about losing Sheer Force for Sand Force, though I guess the inability to equip a Life Orb would've been problematic if it did keep Sheer Force.
We also now know the stats for Mega Sceptile and Mega Swampert, meaning THEY WILL NO LONGER COUNT TOWARDS THE ORACLE CHALLENGE. Sceptile gets 70/110/75/145/85/145, which is just sad considering it's got that glorious physical movepool, yet it gets 40 points in the useless special attack, just like Glalie. Let's be serious, beyond dual STAB, Focus Blast is all it gets. As for Mega Swampert, it gets 100/150/110/95/110/70. WAY better. The stats that needed a boost got a boost, and it just might become the premier physical Swift Swim abuser, thanks to overall better assets than Kabutops. Also, shoutout to MasterLemon for perfectly predicting its stats in the Oracle Challenge.
Now, on to bad news: although X/Y will be compatible with ORAS, it will NOT get patched to be able to acknowledge the new megas, which means you'll only be able to use them against fellow ORAS players, much like Platinum's and BW2's form changes back in the day. I would've imagined given the sheer quantity of them that they'd do something this time, but nope.