I'm a Buffalo wearer
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I mostly wear a base layer underneath - lightweight merino is my choice. They are designed to be worn next to skin - and I have done this - OK. depends on the conditions.
Buffalo is NOT waterproof - but in light on/off showers the wet doesn't penetrate to skin level and your body heat helps evaporate it off.
When it is heavy or persistent rain you will need a protective waterproof layer on top.
They are designed to 'work' best when you are active. When stopped you may need an insulating layer over.
I do not find it warm enough if stopped (lunch) or at camp, especially in windy conditions.
Good zips for venting.
I do not sleep in it - too uncomfortable IMHO - makes an OK pillow tho'. My 'over the top' insulating layer is what I use to boost Sleeping bag.
The Buffalo "Mountain Shirt" is the basic model - I use in temps 0c to 10C. There are lighter versions which I use over 10c, and the "Tecmax" which is great in serious cold/arctic conditions.
They are incredibly durable - my first Buffalo shirt is still going strong after a decade of regular use.
PS. The pockets on Buffalo are IMHO the best - big and in the 'right' places
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