Graham,
"I know people who swear by Leica distos but I'd strongly suggest paying with one before you buy."
Surely NOt paying for one before you buy is theft?
More sensibly, if you meant "pLaying", I couldn't agree more. I have used distos extensively, and I find that the practical problem of aligning teh spot over long distances often takes longer than using a tape. If hand held, spot shake is a problem over 15m, which often results in mis-readings, if you use a tripod, well that takes extra time. I have found however that for taking the passage cross section details they can be ideal - but some racks can prove difficult to get a reading from (particularly wet lava).