I have been using GMail since last Spring when Peter Norvig invited me to try it as a beta tester. Since I use several computers in my consulting business, having a web based email system is convenient - no hassles syncing my email (my local ISP does not support IMAP).
Anyway, one surprising advantage of GMail is the advertisements that are targeted to the text in my emails. I frequently see advertisements for either competitors (of my customers mostly) or interesting information. I don't spend a lot of time checking out competition, but when their advertisements appear along side my email, it is worth a few minutes a day checking them out.