I have both the EF-S 17-55mm and the EF-S 10-22mm (and 5 longer lenses, 4 of them L's). The 17-55mm is on my camera most of the time (and it's the one I leave on the camera when it's not in use so I'm 'ready' to capture family moments around the house).


Point 1: I can tell you that even though I have the 10-22mm, I would miss the 17-24mm range in my 'everyday' lens, and I'd miss the IS, but I wouldn't miss 55-70mm range. For me, 24mm isn't wide enough for some indoor uses, where walls preclude backing up further - I'd rather not have to switch back and forth with the 10-22mm over and over.


Point 2: There's no such thing as 'too much lens' for your camera! Regardless of sensor size, the better the lens the better the images.


Point 3: The overall build quality is a tad lower, from a materials/construction standpoint, not from a quality standpoint. Most L lenses have metal barrels, etc. But I don't think the failure rate of the 17-55mm is especially high despite what you've read. Just like the 'dust issue' with that lens, it's overblown (and even the 100-400mm L zoom is called a 'dust pump', which it's not).


I really think the main point to consider in lens selection are the focal lengths you need, and secondarily the apertures you need at those lengths, and whether you need IS (if you only shoot things that move and are using a short-ish lens, you probably don't need IS). Both the 17-55mm and the 24-70mm have excellent image quality and a constant f/2.8. So it really comes down to focal length. The only other factor is timing of a possible FF upgrade. "If or when" means that's not a factor (vs. "in 6 months I'll be buying a 5D2" for example).


Here's what I'd suggest - since you have the 18-55mm kit lens and the 10-22mm, Look over a fair number of your shots with the 18-55mm, and check the EXIF data. How many of them were between 18-24mm where you didn't change to the 10-22mm? How many of them were at 55mm and you wish you'd had just a tiny bit more? Use that data to guide your decision.


Hope that helps, and good luck!


--John