I am considering getting this lens. I have prime lens in 50 mm, 85 mm, and 100 mm focal lengths. I also have a kit lens from an older camera, EF-S 18-55 mm f/3.5-5.6. It might be worth mentioning that I do not have a fill frame camera, I've got a Canon 40D, which has a 1.6x crop factor.
It seems to me that the 17-40L f/4 would fit the bill under my 50 mm, in that if I did get the 17-40L, I would have good focal coverage for my needs. I hardly ever have reason to exceed 100 mm . All of my prime lens open up pretty wide (1.8, 1.8, and 2.8, respectively), so lower light photography has been very doable.
I have heard and read many good things about the 17-40L with regards to build quality, over all AF speed, and picture quality (though the last thing more so involves the person behind the camera IMO). One thing that I have on my mind is maximum f/stop on the 17-40L. Granted, I do not take pictures at night too often (even then I would have a tripod), or in super dark areas. Is f/4 a decently wide aperture? I suppose I can always crank up my ISO as needed, although I prefer not to go above ISO400-ISO800 if I can. At ISO1600, and ISO3200, things get really grainy, really fast.
Also, I like the wide angle of this lens (17 mm , or 27.2 mm after cropping). I would no longer have a reason to carry around my kit lens, and I would also have coverage of my favorite focal length, 50 mm (which would equate to ~31 mm on the 17-40L on my 40D), which means I would not need to purchase 28 mm primes lens either; which might I mention goes for about $500. So for a ~$200 more, I would have zooming capabilities.
Anyone else have any thought? Art, where are you?