I'd like to share my opinions on all the Megas we have in ORAS, if that's OK.
Groudon: I like its mixed-set potential, and its ability is great, too. Having easier access to Fire Blast and Solar Beam is great for it.
Kyogre: Seems a bit unimpressive compared to Groudon. It, too, gets mixed-set potential, but its typing isn't that great and its expected movepool barely changes.
Sceptile: I never saw the point of Lightningrod. I guess it got better in later gens, tho, so it's not entirely bad anymore. Grass/Dragon hates Ice, but it's got some good offense, too.
Swampert: HULK SMASH! GRAWR!!! Totally uber in rain, too.
Diancie: Mythical 'mon getting a Mega... Sad, since I have limited net access to even obtain Diancie, let alone mega evolve it. It's sure pretty, tho.
Audino: Healer gets a lot of flak, but most tournament competitions are doubles, anyway, so it's not that terrible. Normal/Fairy is good defensively, too.
Lopunny: Playboy bunny'mon does kickboxing. Hawt. :3
Metagross: Warp speed, Mr. Sulu! Mega Metagross is absolutely GLORIOUS!!! I want that shiny event one!
Altaria: Very fluffy. Would make a good pillow or blanket on those cold nights in the wilderness. Dragon/Fairy is great defensively.
Salamence: Reminds me of a starfighter, it's pretty rad. I like how the wings are blades, as well as functioning dragon wings.
Camerupt: Reminds me of those old cartoon animals with the stubby legs, just furiously pitter-pattering their way down the street. Altho this one has a giant volcano on its back.
Sharpedo: I admit, this one seems a bit lacking in inspiration. I like its ability, tho.
Gallade: So knightly! Crap ability, tho.
Rayquaza: Freaking GODLY!!! He looks so much like Shenron from DBZ!
Pidgeot: Such an under-appreciated 'mon, I'm glad he's getting some much-needed attention with a powerful Mega. No Guard is a decent ability, too.
Beedrill: So deadly-looking, it'd be really scary to run into one in the wild, let alone a whole hive of them. Adaptability is a sweet ability, and its stats are epic! Another moldy-oldie made into a real goodie.
Steelix: I feel like it's too over-the-top compared to regular Steelix. I'd have preferred a mechanical design as opposed to this alien-looking one. It's not that great statistically, either, and its ability isn't the best.
Glalie: Its face is locked in a permanent "HELL YEAH!!!" I wonder how it eats with an unhinged jaw? I guess it devolves into normal Glalie to eat. It also has a glorious beard. So cool.
Soopa Hoopa: I know it's not a mega, but that fan-dubbed nickname is so funny. Also, it's got some good stats, and Hyperspace Fury sounds like a devastating attack. The design is so epic, too.