Good addition to the series, but try before you buy.
Graphics - The first thing that pops out at people when they put in SF4 is the new cel(ish)-shaded graphics. I don't know if any of the engines or graphics from Prince of Persia were put into this game, but I definitely had the feel that the new Prince of Persia game and SF4 had a great deal in common. The characters feel cartoony, but not too cartoony. In addition, the graphics all run smoothly and all the super combo and ultra combo mini-cutscenes flow well.
Sound - I'm iffy on the sound in this game. There definitely is A LOT of sound, and even A LOT of good sound; but it's kind of overkill. It goes from J-Pop, to announcers, to screaming, to yelling, to grunting, and so on. I guess what I really mean is that the fighting sounds in the game (the yelling, screaming, general fighting SFX) are done very well, but the menu music and announcer's just drive you insane.
Gameplay - SF4 doesn't really try to reinvent the wheel in terms of gameplay, but there are a couple new nuances to this game which add to the experience. All the combos are the same and if you've played any Street Fighter game before, you'll easily master the combos again. down-over-A…CONSTANTLY. The main new features in terms of gameplay are the revenge bar and focus attacks. The revenge bar is Street Fighter's way of having rubberband physics. Like in some racing games, if you crash early, it's easy to come back because the computer seems to slow down for you. With the revenge bar, if your getting your ass wooped early on, you're attacks will have more damage when they do finally land, FURY IS GIVETH! Basically, fights will stay closer. The focus attacks are kind of cool in the sense that they are semi-unblockable, but that's about it.
Features - One big gripe I had with Street Fighter was the lack of truly different things. There's your standard arcade mode, a time attack, and a survival mode. But that's about it. I would have loved to at least see some sort of tournament mode. The multiplayer is pretty cut and dry too. I will say though that the quality of matches on XBL is very good and there is almost no waiting time in lobbies for a fight and the matches are generally lag free.
Overall - Like I said earlier, this game is a definite rent and a possible buy. If your familiar with the SF series then rent the game first and see what you think. If your not familiar, then I would say download the demo of Street Fighter 2 Turbo HD Remix on Xbox Arcade and see if that's your kind of style.