I've never bought Amazon's because Monoprice's are about half that ($3.50). Monoprice's cables seem to be of great quality too - they're noticeably better than some of the $20 cables my parents have paid for at retailers.
With Amazon's convenience and large selection, it's hard for these niche shopping sites to gain any mindshare on me anymore. When I'm looking for something esoteric I usually check Amazon first and then possibly google product prices to make sure the price isn't substantially cheaper somewhere else. For me to switch away from Amazon at any place that hasn't been "grandfathered" in my mind like Zappos and Monoprice, you'd have to do it via price first, and then keep me with some sort of convenience or other hook.