I too am very pleased with this lens which I use on D3100. It seems like I am bouncing back and forth between this 35mm and the longer zoom while the standard kit lens is used less frequently. As an added bonus I just realized my 52mm filters work with it as well.
My 35mm is also tack sharp with a D5100, but I don't use it for close ups. The other day I had a wander around my village using my D5100 with a Sigma 10-20mm wide-angle zoom. The results aren't that great. I originally bought the lens to fit to a D200 which I no longer have. I had no problems with the D200. What's going on?