I have the Singh Ray ND filter. It's expensive, but one of the best tools in my bag. I have used it to capture speeding taxi cab blurs in daylight, water and waves for landscape, and people walking, running, and biking. It is great.
I am in the process of buying a Lee Filter system because I am tired of buying separate filters for each size lens.
As for selling your brand new 5D, Mark III--Stop reading this forum. Except for a select few, nobody has extensive experience with this camera yet. Somebody was complaining about a photo of their dog not being sharp yesterday. If I read the post correctly, it was heavily cropped, shot with a long telephoto zoom at a relatively close distance, at a F5 (relatively wide open), and the dog was a large one standing at an angle. In short, you have a narrow depth of field, an animal that probably isn't perfectly still, not prime lens, and an object that is not flat against the sensor plane. The photo looked fine on the web (admittedly not the place to be looking).
All of this complaining about the camera is ridiculous. Nikon, Canon, Sony, etc all make fine tools. I see images taken with all of them that I wish I had taken. You can spend all the money you want, it still comes down to the photographer. On top of that, people who are complaining should go see some photo exhibits at art museums. Sharpness isn't everything.