I've been curious about this lens for a while. To be honest, when I had been searching for a do-it-all lens, I simply wrote it off because it was a variable aperture zoom (having used constant aperture lenses, I find it psychologically difficult to go back to variable aperture...:( ).
I initially went with the Tamron 28-75 first (because I found at that time that I used that range most often). Later, when I wanted a wider lens, I got the Tamron 17-50 VC. I like the 17-50 VC but sometimes it's just a bit too short (even on a small-sensor camera with 1.5x crop) and I admit there are times I miss the 28-75's extra reach.
The Sigma 17-70 OS seems to offer the best of both worlds. In terms of focal length range, I think the Sigma 17-70 OS would be great for family photography, effectively combining the focal length range of the 17-50 and the 28-75 I used to have. On a 1.5x-crop camera, this is effectively similar to a 24-105. (Note: this lens only works on small-sensor cameras. On a full-frame camera, Canon has the 24-105 f/4L IS and Nikon has the new 24-120 f/4 VR).
And although the Sigma 17-70 is a variable aperture zoom it's actually not THAT slow. On the long end, it can still go as wide as f/4, which is just one stop narrower than f/2.8.
Apparently, the quality of the Sigma 17-70 OS is good enough to earn the European Imaging and Sound Association's European Lens 2010-2011 award.
See here: http://www.eisa.eu/awards/3/photo.html
Video here: http://www.eisa.eu/award/38/european-lens-2010-2011.html
See also this Photozone review of the Canon version: http://www.photozone.de/canon-eos/517-sigma1770f284osapsc
And the Sigma 17-70 f/2.8-4 OS Flickr pool: http://www.flickr.com/groups/sigma17-70os/pool/
Amazingly, the price of the 17-70 OS is very reasonable, about the same as the *non-VC* Tamron 17-50.
Well, I just got a Sigma 50-150 a couple of weeks ago, so it will be a while before I get another lens, but if I had to choose a wide-to-standard zoom lens all over again, I would take a serious look at this lens and just _possibly_ choose this over the Tamron 17-50 VC.