Pete's right in that you do tend to get what you pay for. However, sometimes, what you really want to pay for is the bargain price.
I got myself one of the cheap variable ND filters a few years ago. Cost me about £12 and I'm happy with the results. If you already have stepping rings, then you won't need the "adapters" - they're basically the same thing although they're more likely to be cheaply made and so prone to thread-stripping, or getting stuck tight.
If you do go for a variable filter, I'd advise getting one with the same diameter as the biggest lens you actually intend to use it on. That way, you can step down a couple of sizes without too many issues. For example, I generally keep a 55m superzoom on the camera so I bought one in that size for the odd time that I might decide to slap the filter on it. However, most of the time, I've attached it to the front of my prime 52mm and 49mm lenses with a couple of step-up rings. I could've splashed out more on a 77mm variable filter which would fit another prime lens but, as I'll never use the filter on that lens and stepping down from 77mm to 49mm would've most likely caused vignetting, it would've been pointless.
The variable filter is also an easier option as you don't have to carry a range of filters with you - you just turn the outer dial to the ND setting you want to use.
