I've started twittering again (tweeting is it?) and have found it to be a. quite useful and b. good fun.
It's (in my mind anyway) similar to a broadcast email, except that it's targetted at people who actually "want" to pay attention to you (followers).
All the tweets with the exception of a direct message end up in your timeline, so you can see a conversation flowing along, if only there was a way to tie replies back to actual tweets rather than just a reply to someone (@user) that would make it a bit easier to follow a conversation between people that you may not follow rather than hopping back through their timeline.
I find it's a pretty good way to expand your circle of friends and meet new and interesting people. If they aren't interesting DON'T FOLLOW THEM!
There are some exceptions, Stephen Fry is Quite Interesting but a twit-flooder, i.e. your timeline could potentially be just Stephen.
EDIT: Silly Me, I forgot to put my twitter name here, it's danmcl0703