The 17-40 f/4L was my first L lens, and it was a great fit on my 10D; nearly equivalent focal length to a standard 24-70 zoom; a perfect walkaround lens. Yes, f/4 is slow, but it's a wide-angle, and I find that f/4 is far less of a problem on wide-angles than on telephoto lenses, simply because you don't need nearly the same shutter speeds.
On my 1DmkII, the 17-40 gets used less than it did on the 10D; the problem is that while it's plenty wide (noticably wider than on the 10D, in fact), it is not nearly long enough; it's a 52mm equivalent on an 1.3x crop sensor. I miss that extra reach, and I find myself going with the 50/1.4 as a walkaround lens on my 1DmkII. I would kill for a 24-70 on this body, since the 31-91 equivalent would suit me just perfectly.
IQ wise, the 17-40 is excellent, and I've not noticed any huge differences in IQ between the 17-40 and the 16-35. I also havent noticed a huge amount of CA problems, as davegregory noted. My copy is nice and sharp, with good contrast and good saturation. By comparison, however, my 70-200 f/4L (non-IS) is sharper, and gives better contrast and saturation, but then I consider the 70-200 to be one of Canon's best zooms. I had a poor quality UV filter on my 17-40 for the first year of its life (before I knew better), but with that gone, it's a fine lens.
All in all, the 17-40 is an excellent lens, especially on a 1.6x crop body, since it acts like a traditional standard zoom. With modern DLSRs producing acceptable images up to (and past) ISO 3200, I wouldn't worry too much about f/4, though faster glass is always nice. When you consider the low price of this lens, you're getting excellent value for money; the 16-35 is twice the money.
If you're using the 17-40 for wide-angle, however, you might be better off with the 10-22, which is slower at the long end and has lesser IQ, but is certainly wider.