I am waiting for the UPS man to deliver my 8-15 this morning. I have always loved fisheye, but this will be my first "circular" fisheye.
What I will spend the weekend doing is seeing if the Full Frame 15mm image produced is better than the Sigma 15mm fisheye I already have. It does of course have the advantage of the zoom, and also the ability to still be a fisheye on a crop camera (I have a 5DII and a 7D), but to me 95% of the shooting with this lens will be 15mm on a FF camera, and that is where I need image quality.
To date I have not seen anyone compare it to the existing Canon 15mm or the Sigma 15mm. You can actually buy Sigmas 15mm and 8mm for less than the 8-15, so the 8-15 had better perform.
I love (mostly) DigitalRev, and would call it something between a review and entertainment.