issues with career mode:
-you can't play it with a real pro, you have to use your created player
-number of players in each tournament are very low compared to the real thing
online play is fine, I've played a ton of matches, you get to see your opponent and their player before accepting a match. I've seen the idiots that try to play with created players maxed out to 100 in every category and just decline the game everytime. Most players online like to use Nadal because he's a good player that can chase down balls most others can't normally get to. I've destroyed Nadal's with pretty much all the weaker players on the pro roster (Chang, Rafter, Wawrinka, Davydenko).
The game plays fair, there probably needs to be more unforced errors but I can deal with it how it is. There are certain players (Roddick, Sampras, Blake) who wield incredible serves that can cause trouble but most of them are pretty crappy overall and you can win in other ways and just get by breaking their serves a few times.
It's a big step up from TS3, if you have Live or PSN access I'd recommend you download the demo and check it out yourself.