I received my Canon EF-S 17-55mm f/2.8 IS USM lens today (12/28/06). It feels heavier by far than the 18-55mm kit lens, but lighter than I expected. Feels good (well-balanced) cradling the lens in the left hand near the base of the lens when mounted to my Canon Digital Rebel XT. I put the lens on the camera right away in a manner that would limit the opportunity for dust to intrude on the back of the lens or into the camera, since I've read about this lens not being sealed as well as "L" series Canon lenses. I may never take it off. 8-)
This lens focuses fast and sure--even in low light, no hunting around to lock onto a subject. Pictures taken with the camera hand-held at 1/10 to 1/15 sec shutter speeds inside with no flash were tack-sharp (Thanks to the IS). With F2.8 and IS, this lens opens up a whole new world of natural low-light photography. Not only are more inside shots possible, you also have the versatility to create gorgeous bokeh. You create these bokeh effects generally zoomed to 55mm (where the kit lens' max aperture is F5.6). Since the EF-S 17-55mm f/2.8 has a constant F2.8 max aperature, you don't have to worry about the depth of field increasing as you zoom. It is so confidence-inspiring to walk around and set your aperture where you want it to give you the depth of field you want, and not really worry about the shutter speed being too slow. Some of my outside dusk (low-light) shots at 1/6 sec shutter and F22 came out sharp (Thanks again to IS). I almost got vertigo when I first looked at my pics uploaded to my PC--they were so realistic. Images really pop!
The decision to go with this lens instead of some of the "L" grade lenses involved the following criteria:
1. Focal Length Range. I wanted at least the range of the 18-55mm kit lens.
2. Max Aperture. I wanted f/2.8, since f/4 is too confining for low-light situations. Also, f/4 doesn't give you the depth of field limiting ability of f/2.8. Trade-off is size and weight (and price).
3. Constant Max Aperture. I wanted a constant max aperture throughout the zoom range. I don't want to set the aperture and have the camera stop it down due to the max aperture decreasing as you zoom in (as is the case with the kit lens).
4. USM. I wanted the Ultrasonic Motor (USM) feature, since this is known to be the fastest and quietest autofocus technology.
5. IS. I wanted Image Stabilization (IS), since this effectively makes your lens faster, because you can shoot in lower light at lower shutter speeds without fear of blur (provided the subject is still). Also, zoomed-into 55mm, camera shake can be more of a problem than at shorter focal lengths. IS has got you covered there as well. This lens only has one IS mode (no mode for panning).
6. Full Format or Crop-optimized. It is true that going with a full format lens would mean that you could use it on any DSLR (35mm film, APS-C DSLR, and Full Format DSLRs such as the 5D and 1Ds Mark II). However, full format lenses are not optimized for the 1.6x crop of the Canon DSLRs Rebel through 30D. The crop-optimized lenses (designated by the "EF-S" in the model name) are tailored to the smaller image sensors of the APS-C camera bodies. The lens elements and coatings are designed to minimize the ghosting and flare that can come from reflections off of the image sensors in digital cameras. Also, the crop-optimized lenses tend to be smaller, lighter and less expensive than otherwise identical full format lenses. I considered the EF 17-40mm f/4L, the EF 24-70mm f/2.8L and the EF 24-105mm f/4L. But, none of these had the focal length range I wanted, and two of them weren't fast enough (f/4), and the EF 24-70mm f/2.8L was very heavy (2.1 lbs). I determined there was no need to sacrifice performance now for some possible benefit later on if I purchase a full format camera. If I ever do, I would hope there would be full format lenses that have been designed to limit chromatic aberrations and introduce other digital optimizations currently provided by the crop-optimized APS-C format-only lenses. Besides, you can always continue to use your old camera and lens as a back-up, or you can sell them to help purchase the new ones.
7. Grade ("L" series or Advanced Amature). Of course, if all else is equal, take the "L" lens with the red stripe. But, all else is not equal. I'd rather have an optically superior lens that is well-built (although not as well as an "L" series) that meets all my other criteria, and just be careful to keep the dust out. The EF-S 17-55mm f/2.8 is expensive (I paid $[...] on [...]). But, I'm sure if there were an "L" series version of this lens, it would be even more expensive.
Conclusion: This is one great lens! There's nothing else out there for 1.6x crop digital cameras that gives you the sweet spot of zoom range, low light capability, depth of field control, image stabilization, fast and quiet auto-focus, and superior image quality rivaling prime lenses. And to put the considerable weight (22.8 oz.) into perspective, it is still 3.8 oz. lighter than the very good digital-only Nikon 17-55mm f/2.8, and the Nikon does not even have image stabilization (and costs more to boot). Game, set and match!