I know that a lot on this board don't really like the Cokin P series, as they are known for not being a true neutral filter and especially with stacked have a noticeable pink/purple colour cast but nothing that cannot be dealt with in post.
I have Cokin P ND2,4,8 and GRAD ND 2,8 I intend on adding another ND8 and GRAD ND4 plus some of the creative filters such as the colours and star filters to my collection over time, yes I would love to have a full collection of LEE Filters but personally I cannot afford that, I would love to get myself a LEE or B&W ND110 and I will treat myself to one in time for my next trip down to the coast in the summer
I also love the idea of the lens rings a very cost effective way of using the filters across my range of lens although I have recently been made aware of step up/down ring converters to use larger diameter filters on smaller lenses
Hope this has helped a little
Matt
MWHCVT