Got one, bought it so that i'd know what I wanted before forking out on ND filters.
At maxiumum ND they're terrible, you get a great big X effect on the image.
At minimum to 3/4, they work ok, but because they're basically two filters stuck together, your chances of lens flare and "weird reflections" on a sunny day are much greater.
They do have their uses, especially when you need a quick solution.
They're worth buying if your not really sure what filters you need. BUT buy a cheap one from China, they're the same item as the branded filters.
I now have a cheap variable, a 10stopper, a grad ND, a polariser and im looking out for a 3(ish) stop ND for my 50mm f/1.8 (to prevent over exposure when using it wide open in sunlight).