A great game with variety of styles. The AI still seems unfair, like many fighting games. Get good at specials to win.
Now, for the grumpy side. I've found the AI to be very formidible, even on the easiest mode. The game offers many levels of difficulty. Even so, I've had a hard time since I don't know the specials and can't do them on command. Simply using basic fighting moves and tactics, the AI is able to sweep the floor with me, easily countering my moves enough to win. Defensive stance works well for many characters, simply waiting to get in a good shot and then pounding hard. Other characters still have good ways of defeating defense, regardless of strategy.
The nitty gritty of the AI is in mimicking your actions. The game apparently detects when you hit an attack button and immediately tells the AI to respond (for those characters who use this mimick AI). This makes a good attack that should work on most humans nearly impossible to successfully execute. This forces the fighter to use defensive stance in addition to special moves. Unfortunately, many characters are able to beat defensive stance quite easily. I've found myself cursing at the screen many times. If the designers of the game had simply inserted a very small delay (mere milliseconds) in the AI's counter attacks in their programming, then this problem could easily be avoided. As it stands, the AI can practically read my mind, countering before I've even done anything. I should note that this happens regardless of the difficulty setting. It should have been omitted entirely from easy modes.
Despite this problem, I've still had a lot of fun. I'm growing anxious to play online and bypass these AI troubles completely. This game genuinely does have a very smart fighting system. In other games, I've found the multiplayer to be unfair, giving advantages in specific situations. I haven't seen any of this in Street Fighter's system. It simply works as you expect it to. As for single player, I admit that the AI was designed with a lot of variety. Unfortunately, it still has one critical problem. Strangely, I've found few fighting games that can overcome this issue, so Street Fighter IV is still better than many others. Give this game a try. You will be impressed... if you go for multiplayer.