PaulTh: You have to be careful of heavy colour casts with ND filters - I bought a Hitech 9 stop ND which cost me £77 and has a blue cast which is very difficult (impossible) for me to get rid of, even from a RAW file in Lightroom.
Phillip Bloom recomends this filter as having no colour cast - I was wondering if anyone else has any experience of this filter. It's pretty expensive but if it has no colour cast then it'd be worth it.
I just came across this thread... Sorry, I didn't get through it all, can you recommend a good vari ND? A specific product with at least 1-5 range?