It's not so much settling as I have not noticed anything bad to warrant getting the Lee.
I once tried using a Cokin 720 P007 infrared filter. However it transmitted too much visible red light. I had to replace it with a Hoya R72 infrared filter.
Two plastic/resin Cokin filters I currently use are the Cokin P120 and P121 graduated neutral density filters. The P120 gives me a two-f/stop gradation and the P121 gives me a three-f/stop gradation. Both give my color images a slight colorcast. The cast is not a big problem with digital images because they can easily be color corrected during post processing. However, the slight colorcast has been a significant problem on some of my 35mm and medium format color slide images.
Who says you have to put the filter in front of the lens?
Unless you want to use grads or a polarizer, you should think about using gels behind the lens if it is possible. It would be a lot easier to carry around and cost a heck of a lot less. You would have to make a way to hold them on the lens though.
Skip the Cokin idea. They warp and are junk.
Warp? Under what circumstances have you experienced this?
My Cokin filters have never warped and they are quite a few years old now. How are you storing them?
They get hot, they warp. Ever been to Death Valley? Desert Southwest? No? Your car can reach the same temps. If you live somewhere temperate then you more than likely won't have any trouble. Glass doesn't have that problem. Kodak gels can warp as well with high humidity/moisture. I have had to toss gels after long exposures near the ocean. Lee polyester "gels" probably wouldn't have that problem but I have heard of focus shift issues with them, especially with telephoto lenses. I have never used them myself so I don't have any personal experience. I seem to recall that Calumet "gels" are Lee but don't quote me on that.
There is the other problem with Cokin filters- since they are soft, they scratch easily.
Like most things in life, if you spend a few bucks more you never have to worry about it again.
By the way, I should thank Mike for the info on the swatch book. I have always used the little ones I picked up at grip houses, but I was unaware that they had larger ones. They are great for balancing on camera flash to ambient light to get rid of crappy color casts. I cut them to fit on the face of the flash and cover it with one of those Stoffen diffusion thingies. With digital it only requires one on the flash. With film, the color cast has to be filtered in front of the lens as well.
I have 2 cokin 8x ND and both have purple color cast.