I've tried a bunch of VNDs because one day I was bored and went to 4 different cameras stores and tested about 10 VNDs against the Heliopan which I own. I did NOT like any of them. Some are sharper than others but they ALL had a green cast. I recently got the Genus Eclipse which has great reviews and I like it better than the Heliopan because there is no green cast, if anything it's a bit warmer look which i like better than green. Also, the Heliopan seems to moire a bit more.
With that said, I avoid Variable NDs unless I absolutely have to use them. I have a Red which has no internal ND so if I'm shooting run and gun, then I'll use them. But VNDs are inferior to traditional NDs which when using a good brand you get better results - no color cast, no loss of sharpness, no weirdness that happens with VND.
I've come to really like the XUME magnets system. It's quite fast in the field. Not as fast as a VND, but still faster than a Matte Box and also screwing in the filter which takes forever.
With that said, I avoid Variable NDs unless I absolutely have to use them. I have a Red which has no internal ND so if I'm shooting run and gun, then I'll use them. But VNDs are inferior to traditional NDs which when using a good brand you get better results - no color cast, no loss of sharpness, no weirdness that happens with VND.
I've come to really like the XUME magnets system. It's quite fast in the field. Not as fast as a VND, but still faster than a Matte Box and also screwing in the filter which takes forever.