I have several Singh-Ray filters, including the Vari-ND you mention (I've also got the warming circular polarizer and a couple of the grad NDs). The Vari-ND is quite nice because it lets you compose your shot through the viewfinder, then easily ramp up the "darkness" of the filter. Optically it seems extremely good.
Here's a test photo I took with it. The photo has flaws, but that's the photographer's fault not the filter's.
Exposure was 0.5 seconds, Vari-ND set to 6 stops. The lens was my Nikkor 18-200 (nowadays I'd use my Tokina 12-24).
There are a few things you should be aware of though.
As I understand it, the Vari-ND is basically two polarizers plus some sort of intermediate filter. This means that you can't use it in combination with a polarizer. Usually that doesn't matter; but if you're taking a shot with it under a bright sun that might limit your options. So what I've found is the Vari-ND doesn't completely replace the need for at least one or two other ND filters (I've got B+W +2 and +6 NDs in my filter pack as well).
The second issue is this. Because of how the Vari-ND works, it's apparently not blocking the near infrared. So, if your camera's sensor (like my D70's) is sensitive to that wavelength of light, you can get some weird effects at the highest setting. Singh-Ray says you can use the Vari-ND for approximately 2 to 8 stops, but with my D70 I can't go further than 7 stops. Still, that's not much of a limitation. Anyway, here's a throwaway I took to demonstrate - note the foliage color changes, and the reddish blotches. Exposure was 4 seconds, Vari-ND set to 8 stops:
That image also demonstrates a third issue - the vignetting problem you can have at very wide angles. If I were buying this filter again, I'd get the slim version - it doesn't have front threads, but I know now that I'm unlikely to need them for this particular filter. Both images above were shot at 20mm; the top one was (obviously) cropped. On my 18-200mm lens, the non-thin version of the Vari-ND will physically vignette until somewhere around 28-30mm.
Now I am insecure enough that I have to point out I can occasionally shoot better photos than the above.
My personal favorite of this fountain wasn't taken with the Vari-ND at all - it was shot using the afore-mentioned B+W 6 stop filter on my Tokina 12-24 lens. But I did have to take the filter off, compose the shot, then put the filter back on (which was annoying, especially when I bumped the camera...).