Ars Technica wrote up a review of various IM client alternatives.
This is the conclusion page. It's all you really need. I'm a pretty big Trillian advocate, but I've been looking at and using gAIM a bit more and more lately. I like it's simplistic design. It seems to operate nice and fast. Although, it did give me grief one day at work, but that could be due to Ford's firewall.