I use a Fader ND with HV30 camcorder and 550D in video mode. It works great for me and allows me to shoot with the aperture I want. So far I've only used it outdoors, I think it's not so effective, or even necessary, indoors.
The only drawbacks I find are that it did not come with a lens cap and that it has no hood to reduce flare. Don't have a solution yet for the hood, but I bought a lens cap for $10.
not sure about the lightcraft variable ND, but i read from dpreview forums that the lightcraft is less linear in demarcating the various stops. I have yet to get this interesting crossbar effect, is it a result of turning past the proposed 8 stops of the lightcraft variable ND ?
The singh ray slim version is actually still quite thick and fat, and will vignette easily on the wider lenses though. I was told the other makes were ? slimmer and lighter. For the SR version, u dun get cross bars going past 8 stops ( but they did mention in their little manual not to go past 8 stops as they will not be able to assure the quality produced ) . But i hardly shoot beyond the proposed max 8 stops anyway.
They have changed the way I have shot stuff on the Canon 5dmkII Before I had to compromise on shutter speed or iris to get the exposure I wanter, now I can get it exactly right with 1/60th shutter in and as wide open or closed down as I want more or less. Also the joy is the ability to gently adjust exposure by turning the variable ND wheel on the front of the lens which gives you what is missing from still camera 35mm lenses, smooth exposure change during a shot. With these you can.