I own a lot of Canon L lenses. In the wide angle zoom line I have owned the 17-40 f/4L and 16-35 f/2.8L II but I sold them both because they were soft in the corners.
After seeing the specs and reading the early reviews for the 16-35 f/4L IS I decided to buy one. All I can say is wow. The image quality from this lens is incredible and the corners are worlds better in sharpness. In fact, the images are so good you might think it was a prime WA. I typically shoot stopped down, so I wasn't worried that it was only an f/4. The IS is very good too so I haven't missed the speed of the 2.8.
If you're in need of a quality wide angle zoom I highly recommend this lens. At this price point it is a terrific lens.