In the beginning I used elm to read my mail. This was somewhat radical, especially as I worked with the team that created POPMail for the mac and Minuet for the PC, and everyone else moved to pine. Then came Mutt -- happy days -- I was able to slice and dice email with amazing speed.
A couple of years ago I converted over to Mail.app -- mostly because of the contacts and calendar integrations, and the fact that I could merge personal email and corp email accounts. In the intervening time I had to move to comcast, which meant running my own imap server proved more difficult than it was worth, so I moved to Google Apps for Your Domain, all of a sudden my personal domain is running Gmail, and I discovered it has key bindings.
All of a sudden it's mutt deja-vu. navigation with vi j/k keys? yes. Single window view (inbox/message)? yes again. Tagging messages? yes. Blazingly fast? you bet. The only thing I miss is keystroke filtering of messages.
That's one reason why I see things like Google Wave working out so well, I might be late to the gmail party, but plenty of folks have been using this as their primary mode of communication for a long long time.