I have a Cokin A series filter system. I like the concept of the system since it allows me to use the same filters for all of the lenses in my 35mm and digital SLRs as well as my large format cameras, but there are a few problems with it. The biggest problem is that Cokin doesn't make a true ND filter. Theirs have a color tint that seems to vary from one production run to the next. I've seen them described as anywhere from gray to purple. This wouldn't be a problem for when I'm shooting film since I usually shoot B&W, but as a filmmaker with a DSLR, it isn't appropriate for me.
I'm also not a fan of plastic. The plastic seems too soft. My yellow filter developed a groove down the center from the lip of the holder within the first few months. I would like some glass or harder plastic filters.
Does anyone make glass filters that fit the A series filter holders? My searches so far have been unproductive.