I do everything; twitter, plurk, pownce and jaiku. Now mind you I use one platform to post to all microblogs, but I check into twitter itself more than the others. I just came off a 6 week hiatus from twitter. I've been on twitter since Nov 2006 and some of the conversations were starting to annoy me, so I took a break unfollowed some folks and now I'm back. There's also a new service called rejaw, which is a cross between twitter and pownce.
I am now a Twitter (www.twitter.com/wifeandmomof3)!! I jumped the bandwagon last night. :-) Since I'm already in Facebook doing it I found an app through it that allows me to update Twitter at the same time so I don't have to do it in over again. This then allows me to reach out to more people to get to know.