Having multiple Macs and a 3G phone, I pipe all my email via GMail and synch through IMAP. Not only does it mean I always have every mail I've sent on each device as a reference, something which POP can't do, but it's also more secure when you're picking e-mails up across the open Internet because GMail sends the authentication credentials in SSL.
The only downside is that my default address isn't my GMail address, yet on certain versions of Outlook my e-mails will show as coming from "Gmail Address on behalf of Default Address". If anyone knows a way to turn that off can they let me know?