Can't say I've used it, but it is a good way for webcomickers, actors, authors, and politicians to use for personal thoughts to be made public. May not work out so well for politicians if they decide to Pull a Reagan/Biden. Businesses have already been mentioned.
Anyone else using it is... well, if they use it for honest fun then good for them. However, most of them are pretty much in it for the attention and the "Next big thing on TV!" shtick. I can't say I approve of Twitter, but it does a fine job of filtering out most of the brainfarts from the rest of the internet and I can't fault it for that.
Now, will I ever use it? No, no... no... no. No... no. No. Nope. Why? I use too many words to say something very simple. Twitter is not made for me.
EDIT: My 666th post... figures that it would be about Twitter.