Hey Sarah,
I can highly recommend the 17-70, its very sharp & has very few problems, you said you might get rid of your 100mm macro lens? if I were you I would keep it for now, while the 17-70 can do macro its not a "True" macro lens, its not capable of 1:1 macro, just much closer than normal zoom lenses, but it is great as a lens that you can keep on your camera pretty much all the time, it does great wideangle stuff, the 70mm provides quite a bit of reach & you can even take closeups, all without changing the lens!
I find that AF has problems focussing at anything closer than about 2" away from the front element, or the UV filter in my case, but the lens can focus closer than that if you go into MF, you can then touch an object on the front UV at 70mm & its perfect focus, although as usual DOF is VERY shallow.
There are a few problems you should be aware of though as its not all rosey, wide open at 17mm there is some vignetting, but nothing that cant be corrected if you shoot in raw & use the vignetting removal feature of ACR (Adobe Camera Raw), its also a little softer at F2.8, but again its about the same as other lenses, where the lens REALLY begins to get razor sharp is F5.6 - F11
I have uploaded a few images which I have recently taken with the 17-70, nothing really stunning but it might give you an idea of what it can do:
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If you want to have a look at some photos that havent been resized, I have uploaded 2 full sized images below:
17mm:http://www.network201.plus.com/Tests/sigma17.JPG
70mm:http://www.network201.plus.com/Tests/sigma70.JPG
If you want to know anything more about the lens just shout

Nick
