MegaMawile and MegaAbsol are okay IMO. However, Game Freak should change their names, like how Yanma's evolution was first named Megayanma, but they chasnged it to Yanmega.
MegaAmpharos is also okay, but I don't think Ampharos should be able to evolve. But it could work as an alternate form.
MegaMewtwo kinda disappointed me at first, but now it's okay for me. I just hope it'll be an alternate form for Mewtwo.
MegaLucario... I'm actually meh on it.
Out of all those Mega Pokemon, Mega Blaziken is my least favourite, because it rubs me the wrong way for two reasons:
1: Blaziken was already broken as it was. It didn't need to be broken any further.
2: Why does Blaziken get a mega form, but Sceptile and Swampert don't? They don't need it either, but they needed it more than Blaziken. Sceptile especially.