Have used these in a few trail runs on mostly desert/semi-arid terrain. Very breathable, fit well and are supportive. Two things I noticed: (1) they breathe so well that they can be a little chilly when a cold wind picks up (for you ultralight mountaineers) and (2) the heel is higher off the ground than I expected (for you barefoot running advocates). These are merely comments. Overall, these shoes have been exactly what I was looking for.