I have to say I really like the plain, functional simplicity of the Google IM client on Windows. It's easy to manage multiple windows, you can easily see where a conversation left off, and there aren't any ads anywhere.
One thing struck me as odd though -- this is a IM client by Google, and yet, I don't see any way of searching through old conversations. There's a way to search for other users, but I'm surprised that a search engine pioneer didn't ship an app with text search built-in.
(Yeah, I know Google Desktop probably indexes Google IM conversations, but still there should be a direct search from within the app)