I was really impressed with this service. Typically with the free services there's a moment where you think "ah...THAT'S why it's free" but I'm still scratching my head with this one.
You can customize your QR code quite a bit, it can be connected to a ton of options, and it even has smart links to route users to different urls based on logic like geo-location and operating system. I was a little hesitant to add them because I don't want them to realize how handy they are and start charging me. ;)
