The Megas are great in fact. So far I've tried the 3 Fire types and they all excel. Blaziken has huge attack and Speed Boost to make hiim unstoppable. Charizard Y is surprisingly bulky on the special side. Not the fastest thing, but with Drought and sky-high Sp.Atk, his Fire moves hit like hell.
But... have you tried Mega Houndoom in the sun? That thing is ridiculous... 140 Sp.Atk + STAB + Sun + Fire Blast + FLASH FIRE if you switch him into a Fire Move. Plus the 115 speed is really nice too. And Nasty Plot... Brilliant.
The only Mega stones you get before E4 are the kanto starter one, Blazikenite (if you have the event), Lucarionite, Aerodactite and Abomasite. Then you'll get your Mega Ring upgraded to detect other Megas in postgame.
I'm now trying the super training on my new Own Tempo Espurr. That's quite fun and challenging. At least more than the regular EV-train xD