Notes on ND filters: (mods - you might want to move the ND discussion here to it's own thread?)
I've used the B+W, Tiffen IRND and LEE system Big Stopper / NDs for long exposure work.
The B+W filters used alone for extended shots tend to allow for increasingly prominent IR contamination resulting in a strong pink/magenta colour cast. For shorter exposures there's no issue. You can fix this with the addition either of a UV/IR cut filter (something like a 486 hot mirror filter) or stacking with a IRND filter. Alternatively what most people tend to do is just convert to B&W as it is perfectly fine as-is if that's your intended output.
The LEE Big Stopper (my one at least) doesnt' have the magenta IR contamination at anything like the level of the B+W and I assume other non-IR cut ND filters. It does still introduce some colour cast but that can be neutralized with a grey card or careful grey point selection. With auto or daylight white balance I find it tends to produce a shift to blue. For daytime shooting I will stack a big stopper and .9 ND pair and wrap the outside with tape to stop light ingress between the filters. This is the combination I shoot when I use my Alpa technical camera.
For my DF/Nikon/Fuji I typically use Tiffen IRND filters. I have a couple of 1.8 & 2.1 pairs in 77mm and 82mm that I will combine to get me to 13 stops. I will typically stack a 77mm and 82mm with a step up filter as this'll work without vignetting with wides such as a 35mm. I've even stacked with a polarizer too successfully for certain situations. The Tiffen is by far the most neutral solution and you can set white balance from a grey card or item in the scene. It isn't 100% neutral and I really don't know if any really are but it's pretty darned close. If you already have a different make of ND filter you can stack just a single IRND with it and you'll effectively kill off the IR contamination that the other filter would otherwise allow. I'm lazy and find it hard enough to remember to put the filter on the lens between shots so I stick with all IRND's and the price is pretty reasonable for standard sized filters. (They get much more expensive as you size up).
You can get your Tiffen IRNDs here (
Tiffen 77mm IR ND, IRND 4x4 Camera Filters and Tiffen IRND Indie Kits) or B&H etc.
Vari-ND filters work great when you're only adding a few stops for use with things like waterfalls or cut light for shooting wide open for DoF etc. When you start trying for max ND settings the cross polarization will kill you with the X bands across the image as stated here earlier. For true daylight long exposure you need to stick to high cut regular NDs.
I'm going to be trying the Tiffen HMIRND (hot mirror IRND) filters soon for use with my full spectrum D800 and also to see if it adds even more neutrality than the regular IRND filters. I still find some level of contamination even with the IRND filters when you go beyond a couple of minutes, although as you've seen with the couple of examples I posted here the look isn't all bad.
Hope that helps.