I've been using it, almost exclusively, since 2005. My original account (crazyrabbits) was part of the beta test, and I still keep it for odds and ends related to past friends and interesting mailing lists. My current account has been in use for 3 years, and pipes in messages from my work e-mail. The conversation feature has been a godsend for when I need to look up information quickly, and I have everything indexed, going back 3 years.
I love it.