I was very trigger happy once I bought my 10 stop ND filter. Any sign of cloud, however high and slow moving (we don't get moody weather here) meant I was looking for a location to try it out! Unfortunately, even with the 10 stop ND screwed in, it often wasn't enough for really high slow moving clouds.
Here's a shot of the Atlantis Hotel on the crescent of the Palm Jumeirah in Dubai in the early morning light. I used a 10 stop and a 3 stop to get an exposure of 2 minutes 26 seconds. Sounds extreme, but the clouds wouldn't have moved much with a faster shutter. Had to push the aperture farther than I would have liked at f22 and ISO down to below the base of 200 on my D90 to ISO100.