At this stage, I've now tested, reviewed, and compared most of the major filters systems currently on the market. The only system I have no experience with thus far is from a company called Singh Ray. Aside from them, I think I might have found the best square format ND filter system on the market.
As someone who shoots architecture, I find that ND filters can be invaluable in many situations. Generally speaking, if I'm shooting an interior I probably won't need to use any ND filters. If there are any reflective surface then I may make use of a polarizing filter at most. For exteriors, however, having a high-quality filter system makes working outside incredibly easy. There are plenty of situations where simply by using a 10 stop ND filter you can eliminate many distracting elements in the frame such as people walking and cars driving past. There are however two main issues that ND filters can on occasions introduce into the image. The first is a color shift and the other is loss in sharpness. Now personally, I haven't seen any real-world situation where a high-quality square ND filter has caused any perceptible loss in detail. For this reason, I think that any loss in sharpness is a minor point at best. Color shifts, however, are a very real issue with most if not all ND filters and this is what I really look at when comparing different filter systems.
In our latest video, Anete and I compared ND filters from Wine Country Camera against the current and very popular M10 system from Haida. In several discussions, I've been told that the Haida filter system was the best both in terms of sharpness and colors and due to that I was very interested to see how they would compare. Performance aside my biggest immediate gripe with the Haida system is the fact that you can't stack multiple square filters or multiple drop-in filters. This is quite preventative in many situations for me. In any case, check out the full video linked above to see which system might just be the best you can buy.