What URL Shortener is Best?

With the limited release of Google’s new URL shortener yesterday, I began to wonder just how many URL shorteners are out there and which one is the best?

Currently Google’s service is only available through the Google Toolbar and FeedBurner, but I imagine they will expand it to everyone before too long…or they may just use it to get more people to use their services.

But that makes me wonder…what is so special about it that one would download a toolbar to use it? What do you look for in a URL shortener?

So far it seems the most popular services are bit.ly, tinyurl (still around), ow.ly, which is what I use through the HootSuite interface for Twitter, and tr.im.? Every now and then I am curious how a particular tweet did…how many people read it or retweeted it, so having the stats from ow.ly helps.

What are your thoughts?