Variable ND is an essential filter for serious DSLR video. Far as I am aware, there are only two manufacturers in the world: Singh-Ray and Light Craft Workshop. The biggest problem is these two small companies are swamped by DSLR demand and are perpetually out of stock. RECOMMENDATION: anybody at NAB should give Tiffen an earful - they are missing the boat. Variable ND was a novelty last year, but an essential today! For more information, see Philip Bloom's comments.
Philip is great, but just because he hasn't recommended anything else doesn't mean there isn't anything else.
Variable ND filters are very convenient, but they are NOT essential, unless you've never heard of matte boxes or normal ND filters.
Tiffen knows exactly what they're doing. Instead of creating a cheap product, they've got some terrific infrared ND filters that do a much, much better job than the Variable NDs. In fact, two polas stuck against each other do things to your image that you often do not want. Think about what polarizers do, and then think about what how that affects your image, like if you're filming people. I have a Vari ND and it never gets used except for rare occasions when I'm taking some landscape stills.