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Singh-Ray Vari-N-Duo (VND with integral polarizer) here. Excellent results with it...no noticeable color cast, nor do I see any other IQ degradation whatsoever. Its what you'd expect from Singh-Ray.
As with any VND, you need to be careful when using this filter at or beyond its max density AND while shooting at a very wide focal length. At these conditions, you will see an X pattern...nothing to be alarmed about, its just the nature of the best. On a FF camera, this usually goes completely away once you get to 35mm or so focal length. You could always reduce the amount of ND to also get rid of this pattern when shooting wider than 35mm, but if your composition depends on it that may not be possible. Just something to be aware of when using this type of filter.
Can't comment on the Tiffen or Fader VND, but vignetting is another concern. I use the slim version of the S-R Vari N Duo, and I will get vignetting from the filter up to about 28mm or just under on a FF camera.