I run Apple's Mail.app and Outlook (for calendaring) side-by side on OS X 10.7 reading from a Gmail IMAP account and an Exchange Account. I prefer Apple's Mail.app - thought I had my doubts for the first few releases of 10.7 when it crashed continuously and got hung up on the Exchange Server with a "constant activity" task. As of 10.7.4 it's (mostly) returned to 10.6.8 stability. It's a viable alternative to Thunderbird/Outlook for mail reading on OS X. Though, honestly, if you are happy with thunderbird, I'm sure it will be many years before you need to consider alternatives.