So glad this lens finally exists. I had a shoot this morning, a very tight bathroom interior, where again (in architecture I run into this often) 14mm was not enough. Forget stitching, I realize that is always an option, but actually it isn't. Time vs money, if client won't pay for premium post processing, no point wasting time. The 11mm would have done it.
I've been waiting for this piece of glass since it was first rumoured. Pulling the trigger on Monday!
You might also consider the Samyang 12mm fisheye + the PS Fisheye-Hemi plugin.