So far there is not a single PS2 online game that appeals to me. I hate tactical shooters (SOCOM), sports games, and FreQuency and TM:B just look odd. Auto Modelista could be great, but doesn't justify the cost of the network adapter.
On XBL, I'm getting a huge kick out of MotoGP, MechAssault, and Unreal Championship. Granted UC is just UT2k3, but the voice and ease of the XBL network is a HUGE leg up on GameSpy and other PC-related issues.
The XBL framework is only really implemented properly in MotoGP (and Whacked) thus far [the friends list and lobby issues in the other games are b0rked], but these were launch titles.
So, being an online gamer since NetQuake, I'll say that XBL is a much more enjoyable experience than both PC gaming online, and presumably PS2 online (due to the lack of.. well, a network heh).
zurich
edit: forgot to add, the banning of modded Xboxs (and thus potential cheaters) is also a VERY, VERY good thing.