Last week I had this lens for a test, so I had some fun with it. I took a few funny portraits of my friends. I'm going to get it back soon, together with Canon 1Ds, so I'll do serious tests later. I'm not sure how serious, because I'm not sure what you can shoot besides photos of skateboarders and funny portraits, which I already did. I heard it's also usable for astrophotography and for paragliders (if they shoot during the flight, they really get everything in the picture). And ofcourse for panoramas.
All the photos are taken in a bar at ISO 400-1250 (800 pushed for 2/3 stops):
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Shot wide open at f3,5 and ISO 1250 (800+2/3 stops in Rawshooter). As you can see, sharpness in the center is excellent.