I use Gmail and it's brilliant for the following reasons:
1. constant updates (unlike Hotmail)
2. unfailingly reliable
3. features like retrospective filtering, POP fetch, auto-forward (standard to mail clients I know but unusual for webmail! also you don't have to have a client running all the time)
Runs good with Outlook 2003 and 2007 in my experience, heard it runs great with Thunderbird on PC. Thunderbird for Mac is incredibly, er, rubbish- as is Entourage- you're far better off using mail.app which is a bit of a case of bashing a square peg into a round hole (google for instructions) but once it's in it's awesome and works fantastically.
