from my experience, adjustable ones are hit or miss. some of them, as you hit its limit, you will get some kind of weird dark lines.
I use ND4 and ND1000, same as gremlin75. I have stacked them before, but in most cases ND1000 is about most i need.
Use them mostly to capture movements in water and clouds.
I would recommend that you go bigger than you need to. 1. to help with vignetting (some less expensive filters sometimes get vignetting on the corners...) 2. you can use those filters on smaller threads using step-up rings.
If you are really serious about using filters on landscapes and etc, you can invest in nicer LEE filters so you can use graduated filters too.
They make circular graduated filters, but you don't have the same flexibility LEE or other square filters offer. Basically your compositions are limited when using circular grads.
Down side of the square filters generally are cost... and bulkiness if you hike or travel a lot.
Don't forget the polarizer to cut through glares!