Useful strength's of ND filters depends on what you want to do with them. If you just need to drop the shutter speed a few stops, maybe 2 or 3 stops would work. If you after doing long exposures of water when the sun is still out, then you may need 6-10 stops.
If you only need a few stops, Lee, Hightech, Singhray are high quality. Some photographers (i'm one) also use split ND filters frequently, lots of threads about those with discussion about pros and cons of still using these or doing it in post (or as I do, a combination of both). Cokin filters are pretty inexpensive, but yes will affect the quality.
Sing Ray and B+W both make a high quality variable ND filter, I"ve used both. They are double polarizers, and can go from 2.5 stops up to 6 or 7. They work pretty well, although I think they affect water reflections ... they are polarizers.
If you want to do some serious ND, I would recommend you look closely at Tiffen's ND filters, originally designed for the video/film industries to solve the issue if IR pollution when using a large amount of ND. These will offer cleaner color once you get to 6 or more stops. I have a 6 stop and a 4 stop, and have used them combined to get to 10 stops several times. They are pricey but I think they give cleaner files.
http://www.tiffen.com/artadams.html