MileCreations said:
is a 'variable' strength ND filter a good idea?
It works, sure. There are various fader ND solutions around. I'd rather a set of Schneider or Tiffen ND filters, but if you can only afford/ only need a fader ND, then go ahead.
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It's debateable, sure. I personally tend to work with Tiffen and Schneider square filters. I prefer Schneider, but Tiffen make decent filters as well.
For example:
http://www.abelcine.com/store/Schneider-4x5.65-True-Pol-Linear-Filter/
Is it a better filter than a $20 polariser off eBay, or even a Polaroid brand? Of course it is. Do you need a better filter? Well, that's up to you.
If you're using the Canon $100 50mm lens, you can probably get away with a $20 polariser. If you're shooting on $4,000 Compact Primes, I wouldn't be using $20 filters...
It's kinda similar to the difference between the three Canon 50mm lenses (ie $100 f/1.8, $500 f/1.4, $1200 f/1.2 L)
The f/1.2L is a
much better lens than the f/1.8 or the f/1.4, with better and sharper glass, a better build quality etc. But it comes at a price, as with anything, and not everybody needs or wants the sharper and better glass.
ND filters
do degrade an image somewhat, sometimes more noticeably. A cheaper filter is going to have a more noticeable effect on image quality than an expensive one.
This is why Roger Deakins mentions in his IBC talk about the Alexa, the idea of shooting 'clean' and how it's sometimes impossible with the Alexa's native 800 ISO to shoot with max dynamic range by sticking at ISO 800, and shoot 'clean' (without filters). It's perhaps much more pertinent, however, with his work because he's using $30,000 lenses - and then it becomes much more of an argument of 'why put a $600 filter in front of a $30,000 lens?'
As long as you accept and understand the consequence of buying cheaper filters, then you can do what you want. That doesn't mean you
have to buy expensive ones, not everyone's shooting stuff that's going to end up on a cinema screen for example.
Just don't complain when you stick in a cheap ND 0.9 so you can get wide open on your $100 50mm lens, and wonder why the image is
so damn soft 
You can also call up rental houses to get an idea of what they think of them - they spend a lot of time prepping their filters, and hear first hand when they come back how they performed, so they'll be able to tell you if a certain filter they rent if worth the money or not. Just make sure they're not run off their feet when you call asking to chat.
