I think that the Sigma is most certainly usable. It´s pretty close to a 50mm in film and I have not used anything in that focal length for ages though so I really need to practice a bit to get a feel for the angle of view again.
Today I went out to play with another lens I recently purchased from eBay.
Those images were taken with a Nikon AF 70-210 f4-5.6D
With the dog pictures I went for a tighter crop to emphasize the expression rather then get the full dog in the frame.