I recently bought a cheap (very cheap) ND400 filter on Ebay for my DA 18-55 AL II lens. I picked the cheap option because I just wanted to play with the effect, but didnt want to spend lots on a filter for a lens I hope to replace soon.
Anyway, back to the point. Colour cast. This is where the quality difference really becomes apparent. The cheap filter produces a very pronounced purple cast. I havent tried to process any of the images yet, it is possible that setting the white balance can overcome the colour cast, but it should not be necessary.
Just another case of getting what you paid for I suppose! I will post some samples of pre and post processing soon.
UPDATE: Shots and details now available.