how is the online play, do a lot of people participate in it? Is it like a lobby or do you need a direct connection to people?
You have 3 major online game types:
World tour - Each week is a new season and there are 4 major tournaments, 2 masters and 1 grand slam which you participate in to gain points. There is also an option to play direct matches where you play somebody waiting in the lobby, but this gives you less points. Beware though as sometimes direct matches take ages to load, in my experience anyway! Also you can only play with your own custom players in world tour mode.
2k Open - You can only play as a pro player and the object is to gain as many points for your favourite player and get to 2000 first, but I don't think anybody cares about that, it's just to play as pro players really.
Online match - Here you can play as a pro or your own custom player and create a match setting or join another (any court, sets).
There are plenty of people to play. Another thing to beware of is that you can sometimes get lag which sucks a bit.
Hope this helps a bit