It's kind of impossible to say how good or otherwise Mega-Mawile will be at the moment. I know a lot of people are singing it's praises - and it certainly has potential - but there's a lot riding on what it's stats are like. We already know that it's attack (with Huge Power) will be great; but it looks like it's still going to be slow regardless. That means it will rely on it's defenses (which are apparently given a boost) - but how much of a boost? Because atm, Mawile has the terrifying defensive spread of 50/85/55. Not even a good defensive typing is going to save Mega Mawile if it's base defense stats don't rise pretty significantly.
That said, it has Huge Power Sucker Punch going for it, along with Intimidate Pre-Megalution. I guess it's just difficult to say anything right now, given the lack of specific information. Which is a totally lame-ass thing to say. But hey, the good news is that we'll have a very good idea of Mega Mawile's potential in about 3 weeks. :O
EDIT: Oh, I just realised that Ev-ing M-Mawile will be a bit problematic too. If you don't invest in defense(s), then you won't be able to take hits. But if you don't invest in attack, chances are you'll be significantly weaker. Just something else to think about I suppose.
ANOTHER EDIT: @_@
Also, seeing as you can only have one Mega per team, it's also hard to compare M-Mawile to regular pokemon; because Megawile won't be competing with regular pokemon for a team slot. Rather, it'll be competing with other Megas for that one valuable slot. So it also really depends on how M-Mawile stacks up against the other Megas more than anything.