Yes, GMail is pretty cool and as Jim notes, it's still cooking in the oven.
The conversation aspect of GMail is really cool - I find it better than using Outlook's categories or conversation thread (although I'm not quite ready to stop using Outlook altogether). What I really liked was being able to create my own labels (just like categories in Outlook) but easily apply them in a web-based mail client. They even have smart addressing (as you start typing the recipients, it pulls down a list of who you want to send it, too).
It's only a matter of time before all the spam really starts arriving to really test out its spam filtering but as with most things from Google, it's very clean and fast too.
Check out : http://gmail.google.com
Preview: Google's Gmail Beta