MGR Revengeance is a decent game. I do have mixed feelings about it, but that's mostly because it botched some of the details. As a whole package, the game is fun, fast, and somewhat inspiring.
Let's get some of the obvious bits out of the way, the story is completely nonsensical. From characters, to plot, to even voice acting, this game really goes bonkers. This isn't necessarily bad, however, as it kind of fits the hack-everything-apart mentality that the gameplay promotes. Still, however, it is somewhat worth mentioning just -how- incongruous things can get.
With that said, this game is all about the gameplay. The only downside I really see to this is that it has quite a learning curve. You may find yourself getting killed by everything incessantly if you go about this game the wrong way. Over time, however, you'll improve your skills and battle becomes much more satisfying. I don't know if it's fair to say the game is 'hard', but it definitely doesn't let you mindlessly mash through the game. There is some thinking involved, and for the most part, that is a good thing.
Funnily enough, Raiden has never looked better. MGR:R was fun while it lasted; not too long and repetitive, and not too short.