I like GameCenter just because it's official and a lot of games will probably implement it. I hate Open Feint because to me it's ugly, it's like a popup ad IMO and it's not very professional looking. For what it's worth the GameCenter logo could use an update. Plus+ has an awesome logo, they need something like that.
I would agree with the earlier comment of preferring GameCenter since it's official. OpenFeint i like second, but honestly, i didn't really like any of them whole heartedly.
Yeah, but it's still in its infancy. Hopefully Apple continues to improve it and add innovative features.
Gamecenter because it isn't slow as balls. The others have more features, but they bog it down immensely. Sometimes simplicity is best.
I Personally like GC because it has its own app and doesn't pop up every-time you want to play a game asking you to sign in.