Thats my oppinion:
AGON Online. It was the first platform I signed and after a wait more news .. I forgot this guys
pros: Can't say didn't tested
cons: they don't have challenges (or I didn't see this)
GeoCade
pros: easy to integrate, very quick in the replies
cons: they only have scoreboards, ugly interface
Plus+ & ngmoco
pros: some features seemed fine, actually they are the unique to offer push notifications for game
cons: only one game, did they care about devs far from them? I think no. Two mails send them (one months ago) and no reply
OpenFeint. Just signed yesterday to see what the offer, but I agreed that for a game/gamer it's better having challenges than chats
Scoreloop
pros: very good features, very quick in reply, they talk with you regarding new features. Challenges it's a must, very easy to integrate. The unique to offer actually the interface in english, german & spanish. You feel the relationship with them to be very close to you.
cons: scoring system it's only by levels (In my game i rather prefer by points, they will work this), no game modes (promised future work there)