I haven't used Apple Mail in at least 5 years. Last time I tried, it was adequate for one email account (which is probably what 80-90% of users have). Though it supported more, things always seemed to go weird after a while if you tried to use that. Maybe it's better now? When I last used it, it pretty much still seemed to be the NeXT Mail.app ca 1990.
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.
