I think it's a great system. I also think that whether it's the right system for you and your group depends on a few things:
- GM Presence: M&M is a very breakable game if the players want to abuse things and the GM isn't strong enough to say, "Sorry, not in this game." While few GMs would admit to being pushovers, it's something that they may have to stay on top of. I don't know how much D&D you play, but assume that M&M, right out of the Core Book, has the potential breakability of D&D with four splatbooks and two additional third-party books full of feats and prestige classes. (Note: This doesn't mean that it's a bad system. It's an awesome system. Its configurability is one of the things that makes it awesome, and it can also lead players to do things that ruin the game.)
- Group Learning Ability: M&M uses a different damage system from D&D and d20 Modern (which may not be what you play, but this board skews toward d20, so that's what I'm assuming), and it can take a little getting used to. If learning new rules is a huge pain for your group, you may be better off with a hit-point-based system (Blood & Vigilance, Four Color to Fantasy, etc), which is more clearly tied to d20 Modern and D&D.
- Combat Crunch Value: This one's harder to define, exactly, but combat in M&M is fast and furious. You know how in D&D, a high-level fight against a big boss and his lieutenants can sometimes take close to an hour per round? (At least, it was that way for me -- having each of the 8 beholders target people, rolling spell resistance, concealment, damage, saving throws, checking the book for immunities, etc.) You don't get that in M&M. The fight that might have taken three sessions in D&D is over in less than an hour in M&M. For some players, who like the nitty gritty tactical detail, that's a bad thing. For some GMs, who are used to having that big combat be the only thing they have to plan, that's a bad thing, too. I don't know that this is an issue worth staying away over, but it's worth testing out some combats with default characters, just to see how it feels. Some people don't like it. (For the record, I love it -- but it does feel different.)
Those are the issues I'd think about. I'm loving the game, and it's working really well for me, but if the above issues trigger immediate "Gaahh!!!" reactions, then it might not be the right game for you.