While I tend use aMSN and Adium for long periods of time, I find that they don't feel anywhere near as stable, and simple, and, dare I say it, elegant, as MSN Messenger 4.
MSN Messenger is fine if you just wanna chat and check your Hotmail email. In fact, in my opinion, it performs those two functions far better than any other instant messenger.
For now I'll be sticking to aMSN (for the emoticons, display pictures, and the 'see-who's-deleted-you' icon) and Adium (for the display pictures and spellcheck support). However, should Microsoft include any of the aforementioned features in the next version of MSN Messenger, it'll be the only instant messenger in my currently overcrowded dock.