The Bottom Line: The Petzl Altitude is an ultralight harness for ski mountaineering that works great scrambling and mountaineering as well. If you want minimal packing size and weight this is the number one choice.
This harness was used for an aborted four-day ski traverse in the Canadian Rockies and ski mountaineering trips bagging peaks. It has also been used frequently while mountaineering as well as on a few scrambles. The Petzl Altitude is a great tool for mountaineers looking to shave weight. It is definitely not a climbing harness, but it is supremely good at what it's designed for.
In terms of comfort, the Altitude is hardly noticeable. Even with lightweight hiking trousers, I've never found it to be uncomfortable. Movement is not inhibited, and walking for 20 kilometers has been perfectly comfortable. Honestly, I mostly forget it's there.
The Fast-Ex buckles make putting it on with skis a breeze. Everything can be done with mid-weight gloves, which is a nice touch. It's so easy that you can put it on even at awkward ledges when you are scrambling and suddenly you decide it might be nice to have a harness on after all. I've done this when scrambling with less experienced partners or on routes where it saves time to rappel rather than down climb. Having something so lightweight and easy to put on is comforting, and it helps offset the added weight of the rope.
The comfort changes when you add weight to it. Compared to the old orange ACMG-strings (Black Diamond Couloir), the Altitude is very comfortable. Compared to a climbing harness, it's not very nice. It's fine for maybe an hour, but you really wouldn't want to use it for hanging belays, nor should you.
The gear loops are pretty rubbish. They are thin material, and hook nosed carabiners are always snagging. They also hang vertically, not horizontally, making things even more awkward. You do not want to be carrying your trad rack on this. But a lightweight crevasse rescue kit is fine. I normally keep the whole kit together in the bag it came with. The saving grace are the clever elastic loops for an ice screw. These hold an ice screw tight against your thigh so it doesn't clang with every step. If you use an aluminum ice screw like I do, this also means a lot of valuable protection.
I really like my Altitude. I've climbed many peaks with it, and it is so small and lightweight that it adds to the comfort on many trips. I like having such a simple and convenient package.
Note: DO NOT LEAD CLIMB IN THIS HARNESS! I have stopped several people from hopping on lead with this style of harness. If you take a lead fall, the plastic leg clips will likely snap, and you will flip upside down. These are designed for low impact falls on snow, not for rock climbing.