I think ND filters, and maybe circular polarizers are about the only filters you still need now that you can do so much in PS. Don't need enhancing filters, color polarizers, color filters (for B&W work), soft focus filters, etc.
There does appear to be some shift in color, indeed. I don't know how much can be attributed to the filter itself, or how much might be the fact that the camera cannot effectively white balance through the super-dark filter. But the shift in color isn't drastic in my experience, and if anything I'd relate it to the effects you might get with a 'warming' filter. Certainly the overall tones warm a bit, so you could manually adjust the white balance towards the cooler side to counter it. But in many cases, the warming is complimentary, saturating and deepening the reds and browns very nicely, making greens richer and less blue, and popping yellows and oranges. I will have to try with a solid or vivid blue subject or sky to see what affect the filter has there - in my other examples I didn't really have too much blue sky or another blue subject. Skies are a handful to try to meter properly with the filter too - the first shot I took of Mission Space was too blown out, so I had to back off the long shutter times a little to pull the sky back into the blues again. Another side-point - if you have a camera that is IR sensitive, this filter does a nice job of cutting out visible light and can help produce IR photos; I bought the filter originally to use with my F717's night vision mode (which removed the hot mirror for IR sensitivity) to take daytime IR photos!
i know that price difference is substantial, but i'd say save up and get the biggest size, 77mm is a good place, step down rings are fairly cheap and you will never have vignetting that way
Good advice, Gary. I am still trying to justify getting a new ND400 in a larger size when I've already got one...but back when I bought mine, 58mm was all I needed. When I got the DSLR, one of my first lenses was the 50mm prime which fortunately was threaded at 49mm, so a step-down ring was a cinch and as you mentioned, no vignetting. But I want a little more range to use it with my 18-250, which is 62mm - so that means I had to grab a step-up ring. I'll be able to use it, but 18mm to around 23-24mm won't be usable without bad vignetting. If I moved up to the 77mm, I could use it on all my existing lenses, and even the 10-24mm and 20mm primes I'm looking at. For now though, I'm still using the old 58mm!