It was only a few weeks ago that we recommended you use Bit.ly for your URL shortening duties, but we've decided to test out another of the major players out there; Su.pr

Su.pr is provided by the popular link sharing site Stumbleupon, and provides short URLs that also come with one of those annoying toolbars when you click on them.

This is not as bad as other applications on the same principle, as it is unobtrusive and provides handy links to the Stumbleupon tools for generating even more traffic to your links.

What is also handy, like the signed-in user version of Bit.ly, is that it provides detailed metrics as to how many clicks your links have received, as well as tips like when is the best time to post a new story (based upon submission times and total clicks).

There is also heavy Twitter integration, and a nice clean interface for users.

We do have a few problems though. For a start, it's pretty new, so many applications don't currently support it, so your followers may not know what to expect from an http://su.pr URL.
Secondly, that toolbar is quite annoying, and you can get similar statistics from Bit.ly that doesn't use one.

From a few traffic trials, Su.pr does seem to deliver a bit more traffic than Bit.ly, mainly due to its integration with Stumbleupon.
This is something to watch for over the next few weeks and months, as rumours have it that Bit.ly is developing it's own similar service to Stumble.

It's a reasonable service, and it'd be best to try it out to see whether it fits your needs.