Widest Lens available for film & FF. Incredibly sharp when stopped down / more than acceptable when used at wider aperature. Great color rendition, solid flare control. HSM is silent, quick, and very accurate, yet has some issues with low contrast subjects (but this lens is not alone in this department either); FTM of the HSM is a nice touch for a UWA, but generally unneccesary - still nice to have. Fit and finish is awesome, the size is great and its weight is a light. Zoom ring is very well damped, making zooming shots (flashzooms and simplezooms) a snap with very little wiggle or bias to the zoom streaks possible. Accepts filters from cokin and Lee 4"x4" systems well, but will vignette from 12-16mm or 17mm because of the hood being in place and the filter holders.
And to the guy asking about his little tunnel vision problem at 12mm, you have to take the lens cap cowl off - not just the lens cap.