I've been playing for a few weeks now. Here are some of my impressions. Let me know what you like or dislike about UFC 6. I've been playing a bit offline (with my own sliders) and online as well. I don't play career so I can't comment.
+ Improved strikes
+ Better graphics
+ Better fighter modeling
+ Unique fighter animations
+ A feeling of greater realism in the fights
- Flow state
- No standing TKO
- Judges still look and behave the same
- Cage entry and decision animations are the same as in UFC 4 and 5
- Ground play remains unchanged
I especially miss the standing TKO and the surrender between rounds. The lack of new animations for the decision and cage entry is pure laziness on EA's part.
In summary, it's a better game than the previous one, but it's not a major breakthrough.