A friend sent me an invite to twitter. I'd heard of it before but, it sounded useless to me. I gave it a try anyway, though, and I'm definitely impressed.
Something I read called it micro-blogging, and that's an accurate description. Posts are limited to 140 characters and it doesn't have a full fledged comment system like a blog would. The system does, however, encourage something like twitter conversations: writing an entry in response to one of your friend's entries. It also has a larger social component than blogging does. I would say it sits in a niche halfway between blogging and IM. I didn't think that was a niche that needed filling until I tried it.
The IM integration is what makes it really work for me. I'm on the computer all day and evening, and most of that time I'm on chat. Twitter will notify me of posts by the people I'm following through chat, and I can post my own 'tweets' in that manner as well. The brevity of the posts and the notification really pulls the content out and integrates it into my workspace.
It's worth trying. Here's my feed.