Street Fighter IV, in simple terms, is the sort of game the franchise needed to move into the next generation of gaming. The presentation is stellar, there are plenty of awesome things to see and do, the gameplay is as good as it ever was, and the game is challenging enough for veterans while still being accessible to new players. Combine this with a solid online component and a ton of challenges to tear through and it's a safe bet this will spend a large amount of time in your console.
The character roster isn't as strong as it could be (though there are still plenty of good characters all around), the balance isn't quite as solid as it should be, the controls aren't as intuitive as one might hope, and the game is missing a few things it ought to include, but at the end of the day, it's worth the asking price. It's fun, well designed and generally good across the board. Whether you're an old arcade veteran or an inexperienced newbie, you'll have a ton of fun regardless.