Ouch. Sure want to use the Apple trio (Mail, iCal, Address Book) rather than Entourage so that I can take advantage of the ease of incremental backup and superior integration with the rest of the OS X (and 3rd party apps) as discussed earlier.
I now believe one of my earlier Mail issues (lost message) was due to “Exchange” mode and some use of WebDAV (on one side of the Exchange 2003 store) and IMAP (on the other side of the Exchange store). This could be mitigated by either 1) using IMAP (rather than Exchange mode), or 2) using Exchange 2007 where there is only one side (again).
The problem is calendaring. The need to interoperate with Exchange Calendar keeps pushing one back to Entourage as the only real solution given Exchange (2003 at least) interoperability limitations with .ics and related formats.
An interesting Google client is MailPlane. It is a front end for Mac users who use Google Mail. It looks pretty good although it has a cost associated with it so one must evaluate it over the free Web 2.0 client.