I am a strong believer in flexibility in photo gear so that I can take advantage of different situations.
I bought a 9-stop ND filter and found it to be too dark for many uses. I then bought a 6-stop ND filter and found it to be much more useful for many things.
After buying the two filters I realized that the best solution would have been to buy a 3-stop and a 6-stop filter.
The 3-stop, and even the 6-stop filters, can be used when you want only a little smoothing of motion. They are also good for outdoor portraiture where you combine ambient light with flash or when you want a shallow depth of field since they let you open the aperture and/or keep the shutter speed below the sync speed.
Stack the two filters and you get a 9-stop filter.
An additional advantage is that the color casts of 3-stop and 6-stop filters is usually very small, and frequently can be ignored.
Even if you have a filter with a color cast shooting a WB card will let you correct the color cast.
If you can't shoot the WB card in the same light as the subject there are multiple ways to correct for a color cast.
Color Cast - Jimmy McIntyre - 500px ISO » Beautiful Photography, Incredible Stories » 5 Ways to Color Correct Beautifully in Photoshop and Remove Any Color Cast
Color Cast - Remove color cast automatically in Photoshop - YouTube
Color Cast - Tim Grey - How to Remove a Strong Color Cast from an Image in Photoshop - LensVid.comLensVid.com
If you use a ND filter be sure you use Manual Focus and do your best not to change the focus when putting the filter on the lens.
If you crank the ISO up high enough you can use Live view for composition. The image will look terrible with lots of noise but remember you will reduce the ISO back down to the desired value for the exposure, which should let you capture a good looking low noise image.
B+H filters were the only high quality ND filters available here in Thailand when I bought my ND filters, and the high import duty made them extremely expensive. After checking reviews I bought HAIDA Slim Pro II MC filters via eBay. The 9-stop filter has an easily correctable color cast, the 6-stop filter has such a small color cast I simply ignore it.
I also tested to see how accurate the ND values were and found out that the 6-stop filter was actually a 6.7-stop filter and the 9-stop filter was actually a 9.8-stop filter.
Haida Slim Pro II MC ND6 and ND 10 Filters - DPReview: Sailorblue Test Results
I am very happy with the HAIDA filters even with the color tints and ND value inaccuracy and will likely add a 3-stop one in the future.
With any ND filter you buy I suggest you check for the color tint and ND value accuracy.
Using a DIY white card or a good quality WB card makes these tests easy. Here is a low cost DIY white card you can use for exposure control or as a WB card.
Sailorblue - Using a DIY Plastic White Card for WB
For determining the ND value of the use the white card. Without the filter find the exposure where the camera's Histogram is almost but not quite butted up against the right edge of the graph. Add the filter and increase the shutter speed by the ND value minus 1 stop. Take a shot at that shutter speed then more with 1/3 stop increases in the shutter speed until you get the Histogram to match the original Histogram as close as possible. The difference in stops between the non-filtered and filtered exposures gives you the true ND value for the filter.
Next take a shot of the card using the camera's meter to set the exposure - the image will show a mid-gray card. Add the filter and increase the shutter speed by the true ND value and take another shot. Set the WB in post with the first shot and find out what the Temp and Tint are for the ND shot.
