I've used it for sometime now and it handles multiple accounts just fine. I run 3 IMAP accounts on it (served from Google Apps) and it handles all of them fine.
Its my favorite email client by far because of its simplicity in function and UI. I get some extended functionality from indev's products, in particular Mail Act-On for quickly sorting mail and Mail Tags. I'd love to see it with slightly tighter calendar integration but otherwise I love it.
http://www.indev.ca/