I finally took the plunge and bought a polarizer for my 17-40L lens a couple weeks ago. I'd been hesitating because I didn't want to spend $170 on a good one quite yet, but I knew it was necessary, and so far, I have loved it. I had one with my previous wide angle lens, but I didn't use it much. I took this one out to the beach a couple days after it arrived in the mail and thought I would share what a useful tool it is.
I am excited to keep experimenting!