[mst]

The Sigma 17-50mm f2.8 OS HSM is pretty close to what that Canon lens does, just cheaper, more sturdy and less heavy. That Canon 17-55mm is not a good example at all "for which one would invest in a system".

And the Sigma is just a little less sharp, isn't it? Seriously: I don't see it as competition really. And I fully understand toni: when I switched to Nikon the EF-S 17-55 was on of the lenses I really missed afterwards. And there still is nothing like it.

To answer toni's question: I'm happy with what's available for the cameras I use. However, I was always very curious about trying out an STF lens in the field, but intentionally never did, just to avoid temptation