Dutch web monitoring company WatchMouse has done a thorough analysis of uptime and performance of 14 major URL shortening services, with quite disheartening results.
As it turns out, only two of these (goo.gl and twt.tl) have had a perfect uptime record between 02/14/2010 and 03/16/2010; the rest of the pack mostly had minor uptime issues, while snurl.com and tr.im had uptime below 99%.
When it comes to performance, measured in the same time period, most URL shorteners can only be characterized as slow. Most of them add more than half a second to the time it takes to open a link; Facebook's fb.me has been by far the slowest, while only Google's goo.gl and youtu.be can really be called fast, both averaging around 350 miliseconds.
Given the popularity of URL shorteners on social networks such as Twitter, this is a big deal. Simply put, while URL shorteners do provide a useful service, they're also making the web considerably slower.