Comment on Adium

Great concept with a very nice default interface and many customizations. Supports the popular IM protocols.

It's very buggy in all .8x versions and seems like 1.x still needs some work. Some of the most problem areas:

dropped connections
loses passwords (keychain items just vanish)
file transfers don't always work
automatic encryption enabling flaky (sometimes it does sometimes it doesn't)
meta contacts sometimes put through a connection on the wrong protocol
deleted contacts sometimes re-appear all by themselves

Still the most usable IM program for Mac OS because of its feature list, interface and ability to support so many protocols.