I wanted to use Mail.app and did for many years but it's handling of Gmail IMAP in Leopard became very slow and buggy for me. It also lost the trademark "bling" sound when new mail arrived. Actually it did it sporadically. Such a letdown.

I tried all sorts of things to get better performance from mail.app and got nowhere so I decided to give thunderbird a shot. Gmail IMAP support is excellent and mailboxes load quickly. I couldn't be happier.

Yes I have lost some functionality but not enough to make me switch back to mail.app. Thunderbird has been a tank in terms of stability and it has speed. I wish it had better Growl support but overall, I'm a satisfied customer.