In reply to Dafydd Llywelyn:
When I had a Nikon camera I found that Sigma lenses were far superior in image quality compared to Nikon lenses of the same price range. I'm comparing budget lenses against budget lenses and not pitting a Sigma against a Flagship Nikon Lens that will cost you your soul.
I've had two Sigma lenses at 17-70mm focal length and they served me well. They were quick enough to focus for me and to my eye, were pin-sharp.
Saying all that, I've moved away from Nikon and gone micro four-thirds and the two lenses I have are incredibly pin-sharp and put the sigma lenses in the shade.
Even though my new walk around lens has a physically smaller focal length, when you compensate for a full frame sensor I sill get a similar focal length to the Sigma Lenses I used to own. I just found that I liked that range for the type of images I take.
I've also got a big zoom. The lenses don't overlap but I either want something landscapy or I want something far away to look big. Think Father Ted.