ManWearPants,
I have used Cokin X-Pro and a custom Lee 'FK102' filter setup on a 17-35N and here is my experience:
The X-Pro is the best off-the-shelf, no mod solution if you want to shoot down to 17mm without hand-holding. The X-Pro filters are much better than the Cokin P-size filters. I have the 3-stop soft & hard grads and I found them to be colour neutral when used individually. When stacking them I noticed some magenta shift in the sky portion of the image. However I have also noticed some colour shift when stacking Lee and Singh Ray but not as much as with the stacked X-Pro's. Long exposures, starting from a few seconds and up, tend to increase the likelihood of colour shift regardless of filter brand. I haven't used the polarizer available for the X-Pro system.
I had Lee make a FK102 holder which slips on to the outside of the lens and is held in place by friction when you tighten the screw that tightens the holder's inner band around the outside of the lens. The '102' refers to the 102mm outside diameter of the 17-35 at the thickest part. Lee cuts the diameter of the FK holder to fit whatever lens it is being built for. The holder has a 0.5mm tolerance which is enough to place a rubber band around the lens to allow for better friction and reduce the possibility of scratching.
I think the FK102 is a better overall solution mainly because it gives you more flexibility when choosing filters. You can use any 4x6 filters which gives you access to ones to which Cokin may not have an equivalent in the X-Pro size. I was able to use up to 3 filters in the holder without vignetting (even at 17mm) because of how the holder sits back from the front of the lens rather than screwing in. For a polarizer, I opted for a 4x4 drop-in that I could place in front of the other filters. The weakness of this holder is that it is less convenient to set up and may not let you be very mobile while shooting. You have to be more careful to make sure it is secure on the lens, otherwise, it could pop off which would be very unfortunate with expensive filters in place.
Regarding filter holder vs hand-holding, I always prefer a holder because I tend to find a composition that I like, do a single setup and take longer exposures. It is also nice to be able to only worry about handling a cable release rather than worrying about how steady I am holding a filter over the course of several seconds or several minutes.
Hope this helps. Sorry for being a little off-topic in an image thread.
Brad