Hi Richard. I have a Lee 0.9 ND and also the majestic B+W 10 stop ND.
It was this shot by Gary that prompted me to get the 10 stop one:
http://www.mysteryme.com/index.php?showimage=300. Some other examples:
http://www.flickr.com/photos/c-l-p/sets/72157603370744181/
http://www.flickr.com/photos/adamclutterbuck/sets/72057594060767730/
http://www.flickr.com/photos/milouvision/sets/72157600209276621/
My method is to tend to stop down to f/16 (or f/8 is quite successful), use ISO50 or 100 and engage mirror lock-up. After focusing, I click to lock the mirror up, whack on the filter and open the shutter with a shutter release cable. Alternatively use auto-focus and then switch to manual focus before putting on the filter. You can't see anything through the lens with it on.
I use a Canon timed remote which enables me to dial in how many seconds I want the shutter to open. It's best to experiment but after a while, you get a feel for how long the shutter needs to be open, and always use the histogram as a guide. If you use f/16 and the 10 stop around dusk, be prepared for exposures of 400 seconds.
I find that the use of the filter has to bring something extra to the image, e.g. I took a shot a waterfall with and without the 10 stop - both images (at 4 and 270 seconds) looked the same.
Hope this helps.