If you need a wide angle lens with superb build quality, weather resistance, great optics, superb colour and little distortion then buy this one.
I shoot a lot of landscapes, sometimes in harsh conditions like the English Lake district or French Alps, and need kit which will stand up to the abuse I give it. If I'm shooting landscapes then 99% of the time I reach for this lens, and if I'm climbing this is the only one I take. What more do I need to say?
I have also used this for group shots at weddings with great results (just ask the Bride!)
It's not light (but not really heavy as it's pretty simple with no IS), and not cheap by general standards (but cheap for an L series), but it does the job and does it very, very well.
I have noticed a little distortion at the very edges when close to the subject, but this is only to be expected at 17mm and for anyone with a crop sensor it probably won't be seen (I use a full-frame 5D). Other than that I have nothing but praise for this lens, and will get another (or it's direct replacement) if I ever kill or lose this one.