I sweat a lot and my old harness wasn't breathable (Black Diamond Primrose). I'd take my harness off and would have big, wet sweat stains around my waist. I climbed all day in the Petzl Luna harness in very warm conditions, took off my harness and I was barely even damp underneath. I was amazed at the difference. The claims about the breathability of this harness are very true.
I do have to agree with some of the other reviewers about the angle of the gear loops. If you're used to molded gear loops that stick out from your harness, these definitely sit a lot flatter against the harness. It took a climb or two to get used to the different gear loop shape, but never had a real problem with them. The design of the loops is helpful when carrying a pack because they don't interfere with your pack's waistbelt. The Caritool loops (for ice climbing) are in between the front and rear loops, which moves the rear loops back a little further on the harness than what most people are used to. But I intend to use this for ice climbing, so I'm ok with it. The rear loops are on the small side, so this harness might not be the best for trad climbing unless you use a gear sling.
Overall a great harness for me and what I will be using it for.