I have a set of Tiffen NDs (.3, .6, and 1.2) that seem to work quite well without being too expensive. I generally favor B&W filters, but opted for the Tiffens for cost reasons and have not found a reason to regret it, yet.
I have had issues with variable ND's negatively affecting the image on a DSLR when used on the Canon 70-200mm. Thus, I try to stick with fixed ND's, if I can get away with it.
Variable ND is definitely the way to go, I think. It all depends on which brand you pick. There are quite a few brands in the market but I think only two have been proven to be the best ones: Singh-Ray and Heliopan. Singh-Ray gives you up to 8 stops. Fixed NDs are just too inconvenient.