Let's see, fair difficulty, insane weapon and attack variety (which leads to ridiculous replay value), incredible and compelling characters, beautiful environments, kickin tunes, badass feel, the BEST cinematic cut-scenes ever, original gameplay ideas - it all leads to DMC3 being THE definitive action/adventure game. The game is hard (mostly the first playthrough) but you'll appreciate that the game doesn't hold your hand or treat you like some chumpy 10 year old. Winning feels really good, as does finally beating the game.
The bad points are few but still there. For example, I hate the sometimes vague mission priorities which aren't clued in well enough, leading to some annoying backtracking at times. I also disliked the fact that some remedial tasks in the game are needlessly difficult, like destroying a statue in a certain way in order to obtain a powerup. Later in the game, destroying these things becomes so annoying you'll just figure whatever and leave it alone. Lastly, even though playing through as Vergil sounds enticing, he's nowhere near as fun as Dante is. Probably because he doesn't use firearms.
There are flaws, but when it comes down to it, This is really the DMC3 they should've released the first time. At 20 bucks, for action fans this game is an absolute no brainer.