I am on Twitter, but I'm a silent participant I'm afraid. I just watch, like a creepy stalker, so not much point following me.
Dr Michael Eades is on there, as is Gary Taubes, but both aren't prolific tweeters. Zoe Harcombe is more chatty. Twitter is great, I love it. For me, it's most useful for getting links to stuff I would never otherwise find. Totally random stuff. Could be a funny cartoon, or an important article, or a new singer, or the announcement of a tour by someone I like, or a breaking news story. You then "retweet" that info to your friends, and the message is passed on, a bit like a chain letter. One tweet can go round the world.
For business it's great for announcing offers, competitions, events, and general advertising, and also for keeping in contact with clients who may be using Twitter socially (to talk about what they had for dinner, yawn). You have to keep tweeting interesting stuff though- Twitter's best when it has an element of the personal to it. Corporate tweets can get awfully dull, and it's no fun if the person behind the tweets never replies when you tweet to them.