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It's pretty good. Corners will be a little soft at 12mm, but not bad. (Not any worse than 16-35mm corners at 16mm). Has some field curvature issues just like the 8-16mm. Isn't the most sharp wide angle available for fx though (like the 8-16 is for dx). I don't have one, but have used one, sorry I have no pics though. Maybe I'll change my 16-35mm out for one at some point if I get bored of it. Prolly not, though. I was going to get one when I switched to fx, but decided 16-35mm + if I needed wider, a 14mm 2.8 samyang, would be a more versatile way to go. Still....
Downside is no filters (unless you rig something, if you search POTN forums for 8-16mm filter you can find my ghetto rig I used on my 8-16). There are some available filter adapters like there are for the 14-24mm but they are HUGE and a bit big for fittin in the backpack.
All in all good lens but not as good as the 8-16mm is for crop, in my opinion. Make sure you check for decentering, same as the 8-16mm!