Hi. I'm cycling from Alaska to Argentina with Avid mechanical disc breaks and after 27 months, I can say they've worked great. I figured before I hit the mountains I was probably on my breaks for not more than a minute or two total per day; in the Andes some of the descents last for hours and proper breaks are incredibly important. I traveled with someone whose rim breaks wore through the rim (an LHT incidentally), as well as with a couple who had hydraulic disc breaks, which boil and start to fail on long descents. I personally won't tour on anything other than mechanical discs... (I also use Rhyno Lite rims for strength, and Phil Wood hubs for field-serviceability).
Paul (
http://www.panamericantour.net)