I actually purchased SQR because I was in need of a link shortener. But boy was I blown away by this tool. It got me hooked from the first login and I've been playing around for at least half an hour enjoying the possibilities this gem has to offer.
The UI is nice and to the point. I like it, and it seems to be getting better even considering the announced plans. The site is fast and resposive (in the double meaning). It works perfect on my mobile device as well.
You can create custom QR codes and really style them to your needs and desires. Codes can be grouped in project folders so you will not get lost in the huge amounts of codes you generate.
A cool feature is that you can repurpose a QR-code, by just changing the link.
They also provide an API, but I didn't try that yet.
In short: go get it. Super deal (no I'm not paid to write this. Just an enthusiastic user).