Good link from Bob. From which you will twig that ND8 is not eight stops, as many people mistakenly think (it's three stops, 2, 4, 8 = 3)
To cut to the chase, the LightCraftWorkshop ND500 is the one I use, as do a few others on here. It's cheaper than most, good quality, almost neutral (none of them is truly neutral) and ND500 is about nine stops (mine is nearer 8.5, but they all vary a bit on that too). Here
http://www.premier-ink.co.uk/photog...t-workshop/lcw-nd500mc/-c-60_365_790_791.html
The X400 is a Hoya I think, and fractionally lighter but all these things are very dark, eg ten stops will reduce a 1/1000sec shutter speed in bright daylight down to 1 second.
There are some tricks to using them (quite a few threads on that here) but the main one is to cover the viewfinder when shooting, to prevent light creeping in through there. Another one that I use is to just count the clicks on the shutter dial to set exposure - saves a lot of mental arithmetic. For example, say my filter is 8.6 stops and my shutter speeds run in 0.3 stop increments, as most do, I set the exposure normally without the filter and then turn it down 26 clicks. I might well tweak it a bit from there after checking the LCD/histogram, but I try to avoid taking the filter on and off all the time which is a right PITA.
You may want to adjust the colour for critcal work, which you can either do in post or do a custom white balance. To save time on location, before I go out I do a custom white balance at home and save that frame on the card so I can reference it and reset any time. Unless the light changes a lot, it will be pretty close.
