Land Cruiserman
I have used this lens for both the Canon and Nikon mount, on the 5DmkII and D300 respectively. The lens focuses quickly and quietly (at least in some part due to what its attached to). DOF is super narrow at 1.4, bokeh is reasonably soft and pleasing without looking harsh like cheaper lenses. Fit and finish is excellent and the lens is sharp in the corners and isnt susceptible to CA or other artifacts of less expensive lenses. Problems: Canon mount: None, its as good as an L-lens, it doesnt have weather sealing if thats a huge deal for you. Nikon mount: My lens front focused. I confirmed this by building an impromptu front/back focusing rig on my dinner table. Sigmas CS was very snarky and rude and essentially blamed my technique and camera and offered to adjust the lens but said it wouldnt be worth it because its the camera. I had my D300 adjusted and cleaned by Nikon, that allowed me to finally dial the camera in to fix the front focus issue, but close to the limit of what the body allows for (16) indicating that even at its best the lens wasnt truly at 0. Testing it out on other Nikon bodies D600 and D800 yielded similar results. Its not a huge deal, since it can now be corrected, but the lens didnt come properly calibrated from the factory. What its for: I like it as a great walkaround lens and for portraits. I use it almost as much as my 50mm 1.4 on my MkII and as a 50mm equivalent for my D300 due to the crop factor. Id dial the lens in with MF adjustment when you first get it to make sure its properly calibrated, and buy it where you can return/exchange it instead of from a retailer that requires you to deal with Sigma.