Slick is the best IM client for 5800 atm. The little notification in top right corner is great. It also uses the least amount of data for updating buddy list and sending/receiving messages. The alert tone is discreet and not annoying, either. Fring is the worst on all of these areas.
I really like the way Nimbuzz works though. If Nimbuzz could let me edit away messages, and have popup notifications, it would be better than Slick. But until then, Slick wins.