I'm on Twitter (@danwallach), but I don't tweet very much. I have it all rigged together, via FriendFeed, such that my Facebook status text and Google Reader "shared" links are all cross-polinated, which also includes Twitter.

I found Twitter to be vaguely useful when I was attending a Usenix conference and various people were tweeting away on the #usenix hashtag. I use TweetDeck, where you can turn any query into a separate column, making it easy to follow these sorts of things. TweetDeck also lets me separate a set of Houston foodies, who seem to have nothing better to do than tweet all day, from my other friends, who are lower volume but I wouldn't want their tweets to blow past me without me noticing.

In short: Twitter has marginal value greater than zero. I fully expect the company to go out of business once they realize that they're not making money and they're not going to. At that point, hopefully some vaguely distributed, interoperable Twitter-ish solution will come out. (A former student and I worked on just such a system.)