Kinda saying the same thing Will Erickson said, but $600 is pretty cheap if your shooting semi-pro or pro.
There's no way a person can shoot in broad daylight with their lens at 1.8 and shutter at 50, which is why you need a Matte Box or Fader. A Genus 77mm ND fader runs for $185 on B&H. If u have a couple of lenses, with different filter sizes, that $600 matte box doesn't seem as expensive anymore.
I don't have a matte box or a fader, which means I usually have to shoot F/11 and higher when I want to keep my shutter slow. I don't like every-shot in a edit with crazy DOF, but I would say shallow depth of field is one of the main reasons so many people are buying DSLR's and with-out a matte or fader you can't really control it. So to end it, I think $600 is total worth for a matte box with rails.