bought the Sigma 17-70 F2.8-4.5 for the following reasons: (in no particular order)
1) It wasn't too expensive ($389 at my local Wolf Camera store here in Texas)
2) It was wider than the 24mm or 28mm zoom lenses which gives me more range. I took test photo's and was surprised at how much more of the scene is available when you go just a little wider.
3) It allows F2.8 (at least at the wide side) which should come in handy when my "walking around" is indoors.
4) It didn't weigh as much as the others in this class, which seemed important to me as this one will be mounted on the camera most of the time.
5) It comes with a petal lens hood
The negatives I've noticed are:
1) The switch to change from AF to MF is very stiff
2) the AF hunts a little too much in low light
3) It is not as fast as my lenses with the USM focusing motor, however, this is not a problem at all for me as it still is fast enough, maybe a little faster than the kit lens. (I'm spoiled by the USM)
4) Bot really a negative, but, the sharpness seems fine to me, not as good as my L lens, but also not as expensive.
Photography seems to be all about compromises, wider vs. longer, F2.8 vs. F4, Sharper vs. not quite as sharp, heavy vs. light and $ vs. $$ vs. $$$ (L). It's just the way it is I suppose.
Overall, I am pleased with this purchase.
http://www.dpreview.com/news/0601/06...sigma17-70.asp