Tried a FaderND, IQ takes a noticeable hit. If you're going to go the vari ND route, I'd go Schneider/B+W or Singh-Ray.
Even with the top quality ones, there are problems at an AoV of wider that 24-28mm (on FF). A vari ND is basically a pair of stacked polarizers, and just like a CPL results in uneven polarization at (ultra) wide angles, a vari ND has a 'Maltese cross' effect that gets worse the wider the angle and the darker the setting.
Definitely go with the Singh-Ray if you're going the variable route, either with the Vari-N-Duo or the Vari-N-Trio. I'd go with the slim versions to avoid as much vignetting as possible. Be aware that even with a slim version at around 24mm you will get some vignetting regardless if you're using a prime or a zoom lens on a full-frame camera. There are dozens of pros out there that swear by these. Personally, I returned my copy of the Vari-N-Duo and use instead B+W and Lee ND filters. I find them much easier to use when it comes to predicting end results. The Singh-Ray filters, even though they have dial markings on the lens does nothing to indicate any true usefulness.
Depends on your use. I'd say for outdoor portraits using a fast prime wide open, a variable ND might be a good choice. The higher end ones will not have a significant negative impact on IQ. But for long exposure landscape shots, where you might be using a wide angle, I'd go with the standard NDs.
