I was against shorteners for a long while, but you can't argue against the fact that it lets you fit more in your message when you have a character limit to deal with, such as with Twitter, identica, FriendFeed, etc. I try to restrict my use of shorteners to microblogging. When following shortened links, I just use common sense and take into account how much I trust the source of the shortened URL. In practice, this has worked very well for me.