As someone with a copy of UFC 2009 pre-ordered from Gamestop, I got the demo a week early. There are only 2 fighters available to play in the demo: Chuck Liddell and Shogun. You can only do an exhibition fight and the tutorial mode, everything else is blocked off. I will go on in depth, but in short, I think the game is excellent.
Standing: The game has a lot more standing depth than I thought. There are a number of ranges to strike from, and you do different things based on the range, button, movement of your fighter, and modification of the strike using the left trigger. The only thing I don't like standing is it's really easy to blow a TKO decision when you have a rocked opponent on the ground and you keep trying to beat his ass but for some reason your character isn't getting over there.
Wrestling and Clinch: I'm a pretty big fan, the controls are easy, and true to life I was much better at getting takedowns as Liddell than Rua. I was getting rocked by Liddell a lot early, but once I learned how to use the Thai plum I was able to dominate with Shogun. You actually whip your opponent around in the clinch a la Anderson Silva to throw the most devastating knees. You can move from the clinch into a takedown as well.
Ground Game: This part of the game is very difficult. Almost too difficult. Moving around to different positions isn't hard, though sometimes I was just spinning the right trigger hoping it would take me to mount instead of north south. In 2 hours of playing the game, I pulled off one submission against the computer, and had one pulled off on me. It's very hard to pull submissions off...that, or I suck at button mashing kind of stuff. I honestly think it might be a little too hard, but then again, as soon as I played a friend submitting him was a lot easier than the computer.
Tutorial: Well worth doing even if it goes a little slow (do we really need a demo of everything?). I found this pretty helpful overall.
When you're playing the game, you can pause and look up combos or moves in different areas, and the list seems longer than a phone book. There is a LOT to learn. I really feel like a complete novice still. I think this game will have appeal on a basic level to button mashers, and some people will become excellent. Still, much like real MMA, anything can happen, and you can get flash KO'd if you're not careful. Especially if you're moving forward quickly and get hit by a Liddell overhand right.
The audio is shockingly good. Mike and Joe really aren't too annoying, and they were basically saying a lot of the same stuff because it was the same fight. However, there was some new commentary each time, and I enjoyed things like Joe mentioning Chuck's fight against Alistair Overeem.
The most challenging part of this is the ground game. Once I moved the computer to expert and fought Shogun as Liddell, he repeatedly pulled guard and went for all kinds of submissions. He'd lock me up in rubber guard, sweep a lot, mount and kill me...the ground game is tough, though I think (hope?) it will get a bit easier over time.
Any concerns I had went away when I convinced someone to come over here and play it with me. The game became a complete blast, and really I just wanted to play as Quinton or GSP or any one of the other stars. I think they are off to a very good start with the series.