I recently got the Tiffen variable ND filter. It's fair to say I did a bit of agonizing over what filter/s to buy. If you go through the reviews of that filter on B&H and Amazon (I don't recall there being many reviews on Adorama), you'll see that most buyers/users report being happy with it, while there are some who report being quite unhappy with it. I'm glad to report I'm in the happy-with-it group. Though I've only used it for one day -- for a wedding.
Of course Jax is right, and I'd love to get one or more of the better performing and more expensive filters. But they are a lot more expensive. And I don't have anything like L glass or such. I was trying to talk myself into getting multiple and more expensive Hoya and B+W fixed ND filters. But that was going to be more expensive, too expensive. And because my priority was going to be trying to keep up with the fast paced demands of shooting a wedding, I went with the variable. There are plenty of reports of vignetting and the X-pattern problem on that filter (which it sounds like is just going to happen with the variable NDs because of how they work, at least with the cheaper ones) at it's darkest settings, but that's apparently (I don't know this to be true for sure) with stills, not video. And video was my main concern. I think it worked out well for me.
Be aware, if you go with a circular filter that just screws onto your lens, you may need or want to buy a larger lens cap to fit it, depending. For example, I bought the 58mm Tiffen variable to fit my 58mm Canon EF lenses. But I had to buy a larger additional lens cap to fit the filter's outside diameter (a 67mm Canon lens cap for about 10 bucks). And because being able to put a cap on the filter to protect it while on a lens was important to me. Tiffen does send along a nice enough pouch-thing with some tissue paper to store it in though.