17-85 vs EF-S 18-55 vs 18-125 vs 100/2.8 Macro USM, actually. Here goes.

WIDE ANGLE

http://www.pbase.com/image/33677837/original.jpg

18-55 @ 18mm f/4

http://www.pbase.com/image/33677841/original.jpg

17-85 @ 17mm f/4 (best of three)

http://www.pbase.com/image/33677843/original.jpg

18-125 @ 18mm f/4

The 18-125 is perhaps slightly sharper than the 17-85 here, and the 18-55 comes in last. The differences are marginal though.

Notice the blueish CA to the left of the text with the 17-85 and 18-55. The 18-125 has next to none.

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NORMAL PERSPECTIVE

http://www.pbase.com/image/33678336/original.jpg

18-55 @ 35mm f/4.5

http://www.pbase.com/image/33678343/original.jpg

17-85 @ 35mm f/5 (best of three)

http://www.pbase.com/image/33678345/original.jpg

18-125 @ 34mm f/4.5 (best of two)

Again, the 18-125 edges out the 17-85 and the 18-55 crutches in last.

The 17-85 is STILL up to its CA thing.

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MEDIUM TELE

http://www.pbase.com/image/33678579/original.jpg

18-55 @ 55mm f/5.6

http://www.pbase.com/image/33678581/original.jpg

17-85 @ 61mm f/5.6

http://www.pbase.com/image/33678582/original.jpg

18-125 @ 58mm f/5

Same story again. 18-125 ahead of 17-85 ahead of 18-55. This is getting boring, except the 17-85 has finally shed its CA problem.

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TELE

http://www.pbase.com/image/33678817/original.jpg

100/2.8 Macro USM @ f/5.6

http://www.pbase.com/image/33678860/original.jpg

18-125 @ 98mm f/5.6

The 100 Macro wins this one, but the difference isn't THAT huge. I had to compensate the exposure of the 18-125 shot by +0.3EV to compensate for vignetting though.

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CONCLUSION :

The Sigma 18-125 is the winner for sharpness, color cast and CA. Also for range and aperture, but we all knew that. An ultra zoom lens that can look anything like the 100 Macro in a corner crop at almost full tele has to be good. We're comparing to what may be Canon's sharpest lens this side of $3000. Remember, the 18-55 fell way behind the rest of the bunch in most shots.

As for contrast, they are pretty much the same, and the 18-55 fails here as well.

The Canon 17-85 is the winner for relative lack of vignetting. Also for IS, and USM with all that that entails.

Build quality seems very similar, and way less dog-toy-esque than the 18-55. Weight and size is about the same, and personally I think the 18-125 has better balance. The 17-85 is flashier to look at, whatever good that will do. The 18-125 looks more like a cylinder, the 17-85 more like an uneven barrel.

What's the verdict ? No lens is perfect. For landscape shooting involving wide angle stopped down, the 18-125 should be the winner. If you want the range, the 18-125 should be the winner. If you NEED image stabilization, then the 17-85 will do but watch the CA and the color cast.

What about me ? I'm going to experiment with CA correction and vignetting correction, and then I'll make a decision on which lens to send back.