I'm needing to buy a set of ND filters. I used to have a rather large set of Cokin filters but gave most of them away when I gave up film photography. I have seen these variable ND filters that work in a similar fashion to circular polarisors, but they get very mixed reviews and can cause colour casts and dark patches in skies. I have also seen a set of Neewer filters which are quite cheap and get decent reviews but they almost seem too cheap ie £20 for a set of four, have any of you guys tried these cheap ones and can recommend them? Or should I just stick to a known brand and pay the extra?