Adium is pretty great, but there's two things about it I dislike, but not enough to drive me away from it (yet).
1) File transfer support is bad (but that's just as true for Proteus). It sometimes works for incoming files, but there's no send file feature that I can find. And there's no dialog box for incoming files, so you don't know if they're working, and have to wait for your buddy to tell you.
2) Developers are hesitant about features as "options," and usually turn down any suggestions that they personally don't like, even if several users would like to see a feature offered.
Other than that, it's a great program; I like how customizable the buddy list is. I've gotten it the smallest I've ever seen a buddy list, quite a good thing when your monitor is only 1280 x 854.
--Cless