I've been using Proteus for the past month or so. I've been running the latest betas for 2.0 and really like it. I think it's definitely the better multi-protocol IM client for OS X. I highly recommend it, the latest version even has tabbed conversations. RC1 of version 2.0 was just released last night. It sounds like his next version will also be targetted for use with OS X 10.2.