I was a Thunderbird user for 2-3 years, but the lack of Blackberry synchronization, the inability to use Outlook-specific plugins, the loss of integrated calendaring, and no effective support for Windows Desktop Search brought me back to the Outlook fold (for better or worse).

The beauty of Gmail is that all of your mail is archived by default, even when sent/received from a "fat" mail client via SMTP and IMAP/POP. So regardless of which mail client you use, all of your inbound and outbound messages remain searchable via the GMail web interface (and on a Blackberry, through the Google Apps GMail client!).