The Canon 17mm f/4 TS-E is a fairly new lens (announced last fall and only now trickling into stores). I was fortunate enough to get a copy and finally had a chance to test it out on the 1Ds Mark 3.
It is simply put a stunning piece of glass. The build and construction is absolutely top notch and has a quality feel to it. The lens is considerably larger than my 90mm TS-E and the large bulbous front element is mesmerizing. The amount of shift possible is also quite amazing.
I did a bunch of boring test shots to evaluate sharpness and CA across the frame when fully shifted. Sharpness is excellent and by f/8 even the extreme corners are very sharp when fully shifted on the 1Ds3. CA is also very well controlled. There is moderate vignetting when fully shifted at f/4 but its well under control by f/8.
Flare will be something to be careful about. In the image above the sun was 90 degrees to the lens and yet there is two (albeit small) flares in the image. Curiously there is almost no flare when the sun is in the image itself.
I only had the chance to take a couple of images at work before heading home. One of the things I really like is the ability to correct perspective distortion. In a lot of urban shots with an ultra wide lens, you end up getting a lot of perspective distortion. Now, you can find a way to use that distortion to enhance your composition but having the ability to compensate for it is definitely very cool.
I know that Canon gets ragged on for their ultra wide angle lenses, but from what I've seen of the 24mm f/1.4L II and this 17mm f/4 TS-E I think we have some excellent high quality ultra wide angle choices.