The beauty of wide-angle zoom lenses is their flexibility, allowing real control over shooting perspectives. This wide-angle zoom is a very high quality but moderately-priced ultra wide lens making the Canon EF 17-40mm f/4.0 L USM one of Canon's best selling lenses, according to their own website.
It is a great looking lens, emanating solid, robust build-quality and that additional red ring gives it a distinctive look. With so much technical information and specification site available, there is a little point to going into them again here; the product information details are a little sparse with Amazon and I recommend going elsewhere, e.g. Canon's own site, for in-depth specs. Although obviously written by partisan Canon employees, while researching punctiliously, I read it before buying and I cannot think of anything with which I disagree now that I have one. On 17mm, especially with the 5D Canon SLR or better, this lens is stunning
Although it is a solidly built lens, it is also relatively light, making it a sensible choice for lots of assignments or photograph types or those holiday snaps for the real enthusiast. I have used it in many situations and never been disappointed.
I recommend using the Canon EW-83E lens hood for safer shooting; although it is quite a wide hood, it is very good and does the job excellently.