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.