Just did a fresh install of Leopard and installed Adium. It's kind of driving me up the wall but I'll add an alias for a user. It'll save it for the time being. Close out Adium and re-open it, alias is gone, full name is back
1.2 specifically fixes some Leopard issues (most notably for me was pixellated icons) so it should work fine. You may want to try archiving your preferences if you're having issues.
No performance issues, but there are loads of bugs that are miraculously disappearing the more I use it. I'm hoping the bug that causes Adium to lag after I've used it for too long is no longer present, but I haven't been chatting much lately.