I for one advise against them... I myself bought a cheap one off of ebay in order to increase my exposure by the most i could. The reason i went for one of these is, i ended up reading that even high end ones tend to give you that 'cross' (cross hatch?) look, although some have a stopping mechanism so that you can't fully rotate them (as the cheap ones do) and so you don't ever notice when it starts or stops. The cheap ones do degrade the image noticeably, and i fear i'm sure the expensive ones do as well, perhaps to a lesser extent.
I recommend only using 'single density' nd filters as they are many times better.
I ended up getting (again from ebay, reading reviews this time) an equivalent 10 stop filter, which actually does the job. For black and white it is great! Unfortunately, doing my exposures for several minutes i don't calculate reciprocity failure very well, nevertheless, it has been very fun to use it, especially the times it did work. I went for the 77mm as it fits my mamiya lenses as well.
If you are to use one with color i would be careful with color casts (as a user of dig*tal i can fix this specific filter with large amounts of white balance, but for film, i can't imagine the work it would take).