I have the Sigma 17-70 OS HSM and like the lens very much. I have had different lenses in my past, primes and zooms. I somehow always end up keeping it simple and having one zoom that covers the most used focal range and this Sigma has filled it's purpose fine. I find the lens sharp enough, it's bokeh is very nice, very much consistent with other good Sigma's and it's OS is very effective. I had before a Pentax K20D which has in-body stabilisation, and this lens has better stabilisation. I use the lens primarily as a constant aperture f4 lens (I use aperture priority most of times), going to other apertures if needed, but this way I do not need to think about the aperture change. f4 is very acceptable for the whole range and the bokeh is not getting bad at this aperture.
I have some pics on my flickr stream, all the recent pics are taken with this lens.
I hope you make your choice wisely, so you do not need to pay again to get some other zoom to fill this range. I personally took this lens happily over the Tamron 17-50 VC for better range, better reviews, good bokeh and the lens also had "better feel" in my hands and in use than the Tamron (and looked better and even sounded better . I personally would not choose a zoom for Canon without IS (or OS).
70D. EF 70-300mm L, Tamron 90mm F2.8 VC and Sigma 17-50mm F2.8 OS HSM.