Who's using Sigma 17~70 2.8 EX DC OS HSM
There is no such lens as 17~70 Sigma F2.8 to my knowledge. There is a variable aperture lens Sigma 17-70 2.8-4 OS HSM. I have owned it and used on D5100 for about a year. I was not very happy with it, my copy had usable sharpness at widest aperture only up to 35 mm above it had to be stopped down to get some decent results. There was lot of hit and miss in focus. Ultimately I sold it out and bought Sigma 17-50 2.8. The new lens had a very bad focusing issue, did not focus at all. I got it exchanged from retailer. The replacement is really good, Sharp at all focal lengths , slightly less at 50 mm. Focus is quick and accurate, No problem of soft images while shooting at wide open at all focal lengths.
My experience of buying 17-70, was no upgrade over kit lens except extra 15 mm that I did not need. However I have seen many happy users of this lens here and other forums as well.