I have not had any issues with this lens what so ever. Auto focus works flawlessly. These photos were shot in FF not cropped to x1.6 so its a true 15mm on one shot and 16mm on the other shot.
You can use this lens on a FF camera at 15-16mm without the heavy vignetting. You can also set up your R to x1.6 crop and use the entire range with no problem.. And not being an EF-S lens it wont force the camera to auto crop which I think its awesome..
I have used my 550d kit lens EF-S 18-55mm and not only it force crops but you will be shooting at 11 or 12mp out of this camera.. Not sure why the loss of MP..
The crop is a 11.6mb file Isaac, from the centre of the sensor. Having the 1.6 crop to flip on when ever its needed is kinda cool.
Sony is the same such smaller file size in Super35 mode