dont go for the really cheap ND filters. i had two of these, and one of "medium priced" ones.
the 2 cheap ones both had "hot spots" (spots where the coating was fucked and light leaked in), so i returned them.
if you really want to get into the whole filter business, it would be best to stay away from the screw-on-filters and go for a system with filter-holders and stuff.
every one i know that is into landscape or similar stuff swears by them.
kinda expensive in the beginning though.
picked up these 2 gems in stockholm recently, really stoked on the fred mortagne book, it was on discount for like 10 euros, plus my girlfriend picked up a great miles aldridge book for 30 euros.