I've generally found Cokin filters to be fine. I use coloured ones for black and white on my RB67 and 35mm, red, orange, green and also infra-red. The grads are supposed to be a bit iffy, not neutral, so I have Lee for those, thought they are expensive and I think there was a bit of a waiting-list for them. The Lee holders are pretty good, and you can get wide-angle adaptors for them which makes the holder stand out less far from the end of the lens reducing the issue of vignetting. The self-supporting lens hoods are good too. I would say if it's an occasional use of filters, Cokin is great, you can pick up a holder and a pile of filters for next to nothing and I think you'd be hard pressed to find a quality difference on the print. Lee is an investment, but it's a good quality product, hence the price, and I think many people buy into it on the strength of the reputation of their grads.