I left this until last because it's the tool I dislike the most. But I summoned up the courage and opened a twitter account. I can see the appeal for those who enjoy texting, or IMing, but at this moment, I have no need for such a tool. We did use IM frequently while I was working in Tech Support and it was a very handy way to communicate with someone while on the phone with someone else- a frequent scenario in Tech Support.
There are lots of widgets, and ways to get Twitter on your cell phones, etc. but I have confined myself to viewing the public timeline only. Then I will probably delete the application altogether.
But that said, it sure is easy to join and easy to use- the hallmarks of Web 2.0: fast, free and easy!