So I've had this lens for about 1 week and all I can say is WOW!!!
I've been using the XT for about 8 months now with the Sigma 17-70 and I thought that was a good lens, but found it much too slow in low light.
I've found the 17-55IS to be THE ANSWER to everything on a crop camera...Great for all-around walking around, great for low light, great for portraits, great for landscapes. In fact since getting this lens, I have posted both my 50mm 1.8 and 35mm 2.0 for sale as I know they will NOT be getting used at all anymore.
I will say that the lens is on the bigger side on the XT, but I leave my grip on and it all balances out.
I was able to compare the Sigma 17-70, 17-55IS and 17-40L in detail for a short while, and even though I know the limitations of the 17-70 and was using the 17-40L on a 30D, I was still able to get sharper pics right out of the camera with the 17-55IS. The 17-40L did have much truer colours I felt, but it couldn't compete with the 17-55IS in low light.
This lens really should've gotten the L designation, (as should've the 60mm MACRO), but for whatever reason Canon didn't, and didn't give it the weather sealing and rugged body composition. Doesn't matter, this lens should be seriously considered for those with the $$ and using a crop camera.
Given the size and weight of the L competition (24-70, 24-105), the 17-55IS makes for a better allaround lens. The 17-40L is just too short and not fast enough. Don't even try to compare the 17-85IS anymore, the 17-55IS will get you much better looking pics, while staying moderately light and giving you both IS and fast speed.
Canon is on the right track with this lens. I don't know what they were thinking when they brought out the 50mm 1.2L and 70-200 f4IS, what they need is a 17-105 f2.8IS, or 17-85 f2.8IS to compete with the Nikon and Sony kit lenses for reach and inbody IS for Sony.