Currently I use a B&W Split ND filter, and am quite happy with it. In fact all my filters are B&W and I have no complaints.
However, I remember years ago I also had Cokin filters for B&W and split ND. The obvious advantage for split ND was the ability to move the horizon. When I look at those old slides (mostly shot with my old 28-70 L lens), I don't see any degredation in IQ.
However, I was wondering with new model digital cameras is there a loss of IQ by putting a cheap piece of plastic in front of your $1,500 lens? I've always used the B&W filters because the provide the least degredation to IQ, but what are your thoughts on Cokin?