I've been playing this game for a couple of weeks now, and, in my opinion, it's not bad. The fight mechanics take some getting used to, and you should not expect a fast paced recreation of the UFC. This game is a relatively slow and tactical fight sim.
Major flaws:
Clinch and ground game are still not very realistic. I mean, seriously, how easy is it to get out of mounts? And if you're on top of your opponent, in his guard, there's no way it should be that easy for him to escape. Honestly. Clinching can end up with what appears to be 2 grown men gently cradling one another in what is, I guess, attempts to gain a dominant position.
Difficulty curve is pretty steep. Playing through career mode, on easy you'll find yourself beating up the top guys very quickly, with no problems. On experienced, however, the jump between top ten and lower ranked guys is just immense. In my experience, you can be owning all these lower ranked fighters with regular first round KOs or TKOs, but as soon as you fight anyone a little higher than that, they're all over you, and you've lost within the first round. There should be more of a curve between the 2.
Aside from that, and the slightly out of date roster, it's a pretty fun game. Definitely worth the £13 I paid for it.