Who does? I do. It's pretty cool, a bit like the status thingy on Facebook but with an RSS feed, mobile alerts and you can update it using AIM or Jabber.
www.twitter.com
I only tried it because TUAW were spurting over it repeatedly last week. I'm pleasantly surprised at how addictive it is, although it's semi-useless. Poke me if you get it, I'm Spikespencer.