Breakthrough Photography has introduced their new X3 line of ND filters, claiming that "The X3 ND is the world's sharpest and most color neutral ND filter." For validation of that big claim, they sent me a pair of filters in 6 and 10 stop densities. I have included the 10-stop X3 in many of the comparisons shown in the 10 Stop Neutral Density Filter Review and have added more specific X3 information at the end of that page.
This fact should grab your attention: The X3 is less than half the price of the Singh-Ray Mor-Slo 10-Stop ND Filter. When I get time, I will update the neutral density filter review with the results from the 6-stop filter, but ... this filter performs similarly to the 10-Stop version. Excellently.