Sigma primes are surprisingly good especially with their bokeh. The new Sigma 24mm f1.8 is a screw drive only and is designed for full frame cameras but will work with some select DX cameras (auto focus wise). I'm sure the Nikon lenses might be slightly sharper on corners but in real world practice, they are very much acceptable.
Sigma 85mm f1.4 and 24mm f1.8 lenses by gqtuazon, on Flickr
Size comparison
Primes by gqtuazon, on Flickr
Sample image using the Sigma 24mm f1.8 lens
Sigma 24mm f1.8 Iris by gqtuazon, on Flickr
Sigma 85mm f1.4 and 24mm f1.8 lenses by gqtuazon, on Flickr
Size comparison
Primes by gqtuazon, on Flickr
Sample image using the Sigma 24mm f1.8 lens
Sigma 24mm f1.8 Iris by gqtuazon, on Flickr