There are several comparison tests if you google zeikos. Very positive.
I use Zeikos for my video cameras, 72 & 77mm. I shoot a lot of outdoor events and I have to pretty much consider my skylights as throwaways every season, they just take too much abuse (better than considering my camera a throwaway!) The Zeikos is just fine, and a skylight is one filter you don't need the color accuracy of a good ND vs. a cheap one with a color cast.
The Zeikos circular polarizers are also great products; for my big broadcast "video" cam (built-in lens, XLRs, etc) I only use a skylight, polarizers, and occasionally a graduated ND. No color shift problems with the Zeikos, I think they're in it to win. (For really "pretty" cinema-look shoots I use DSLRs with 4x4's in a matte box. Wish Zeikos would get with the 4x4's.)