I was hoping this lens would be a good transition lens good on both crop and FF. it's wide enough to use as an UWA on crop and even better on FF once I feel like upgrading. However at $950 it makes much less sense. It would be great at $500. For crop the Tokina 11-20 and 14-20 make more sense still.
I see that a lower price is suggested for the upcoming KS program, but not much help if you don't want a lens relatively soon.
Having tried the lens on my 100D as well, it's wide, but Canon's 10-18 is wider and you have a lot of other wide options for crop cameras.
That said, never do I feel the difference between crop and full frame more than when I put an extra wide lens on the camera. The differences between 12mm and 17-18mm (equiv.) in composition and approach to shooting are much more obvious than the few mm might seem.
One reason I enjoyed testing the 12mm was that it forced me to think wide in a way that my Canon 11-24 didn't. Over the last year my shots with the 11-24 had drifted narrower, but the 12mm has given my creative processes a good prod. Given I do photography for a living, that's never a bad thing