indeed, grit getting onto the pad surfaces and making hellacious noise was my biggest complaints about the shimano hydraulic disks on my stumpjumper. as brakes, they worked great, but I really don't like noisy brakes, I like my bike to run silent.

but heck, I adjust my v-brakes pretty regularly too, keeping them just so... not too much gap that the lever gets mushy, but not so close they scrape, and just the right amount of toe-in so they don't squeal.