I own one of these. Absolutely love it. It's so far been pretty dirt/water resistant. Most just rubs right off. I've used it in light rain and had no issues. Thought after reading PDX's post above, I might spray mine down with a water proofer for added protection.
My biggest concerns with it are accidentally stepping on the plastic clips when it's unrolled and breaking them, or it blowing away if I'm feeling lazy and not weighting it down.
Other than that it's been a great investment. I got it during their last sale that made it like, $25-30 or something.