I've dabbled with vari ND's, and I have not been that impressed. The performance on my 18-50 was ok until the cross tipping point, on anything wider or more telephoto the ones I have used have been horribly soft.
If I could find one which would work on my 70-200 f2.8 I'd pay top dollar. I can see they would be very very handy for my video work, but I'm not willing to drop the money on one based on my experience so far.
I prefer using square filters on the front of the lens, though never stack ND's as the weird colours start kicking in.
This does mean having an ND2, ND4, ND8 solid and then ND grads as well, but the end result is better, so for me it's a no brainer.