I have literally used all the chat clients out there for Mac. I have used aMSN, and I found the application to be very buggy and slow for the most part. Aesthetically speaking, of course, aMSN just isn't very pretty to look at. If you really need a client that supports MSN, then I'd highly suggest Adium. All the customizations may scare you at first, but I feel that it is the best client.