I bought this lens to have a fast f2.8 wide angle zoom to complement my Leica 135mm f2.8 that I normally use in club & concert settings. Most of the venues have less than adequate light and I'm forced to shoot at ISO 1600 or higher, at f2.8 at shutter speeds at 1/60 or less (mostly less)
Because of inadequate lighting, I used the lens in manual setting (it took me awhile to understand that I had to pull/push to change from auto to manual), I have found the lens hard to focus (even with the focusing screen in the LX) and also have found that the results using it at f2.8 gave me a soft image. I have reverted back to using my old Pentax A-SMC 35-70mm f4 at even slower shutter speeds and sometimes higher ISO ratings.
I must admit that I haven't actually used this lens in other situations so can't comment on how it functions in "normal' use. Also I haven't used the lens at higher aperture in club use as I
already have my trusty older manual lens that gives me the quality I want.
I originally bought the 28-70mm Sigma EX lens to use with my Pentax cameras, specifically in dark club settings as a wide angle complement to my Leica 135mm F2.8.
Unfortunately, I was unable to use this lens with my Pentax LX as it didn't have an aperture ring. If, at the time, I had had my DSLR, possibly I would have kept this lens as it was smaller, seemed easier to use, and sharper, and about $100 cdn less.