I just got the non-OS 17-70 as an upgrade for my kit lens.
I was also contemplating the 15-85, but in the end, I just could not justify the additional cost.
Only had the lens for a few days and I am very happy with it.
First few initial shots, I was still on the fence. My first shots were blurry and/or oof.
Next I put the camera on my tripod and took side by side shots with my kit lens and was quite happy with the results.
I think the first shots were operator error.
When I stopped both lenses down to f/9, the Sigma blew the kit lens away.
I was also nervous about "focusing issues', but mine worked great out of the box.
So the pros are:
I gained a little more light, f/2.8-4.5.
I also gained a little more IQ.
I gained a little more on the long and short end.
I also gained macro capabilities. (Just starting to play with this.)