I've always been using aMSN. I've probably been using it for the last 6 or so years. Main reasons for using aMSN over everything else;
video chat, offline messages, tabbed chats and displays who has you deleted on their list, great logging, custom emoticons! etc....
I won't use Adium, simply because its a joke; slow transfer speeds (3.5kbps), does not display all avatars immediately or at all, msn names are not always shown properly to people on your friends list, no web cam or audio support, no offline messages (i dont think at least), custom emoticon support is *really* poor. It's far worse than regular msn for mac. The only thing saving adium from being a complete msn failure is its very nice UI. However, its not enough when the application isnt functional.
Each release of msn for mac only adds a few, at the very most, new features and it takes a year at least before a newer version is out. It's a joke as well. The only guys doing anything right with MSN for mac are the guys behind aMSN. The problem is with them, is that its a bit of a slow application with some minor bugs, the good thing is that they dont crash or interfere with chats, just some UI bugs. I've got a rather nice skin theme for aMSN, so it doesnt look horribly bad;
Now, there is also TRILLIAN ASTRA thats supposed to be coming out for mac sometime in the 'near' future. I say near loosely because it took them way too long to release for windows, and since december's blog posting in 2007 about the mac version theres only been ONE announcement made since then, this year in june and had more to do with the iphone compatible release. Now, the Windows version of Astra looks pretty promising, hopefully they'll do a similar job on the mac side.
Read about Astra here:
http://blog.ceruleanstudios.com/?cat=13