The idea of buffalo is that you wear it as a single layer next to the skin, nothing on underneath. The pile then wicks the sweat off of the skin and towards the pertex. The pertex spreads the water about over a large surface area, meaning it drys fast. Its the tips of the pile that make the most contact with the skin, these dry very fast and that's why even a damp buffalo can feel dry. I usually wore mine with a good synthetic baselayer in case i got too warm, that way i could still take it off without any hassle, the baselayer didn't seem to affect the wicking that much but i wouldn't wear it with a cotton t-shirt or anything like that.
Buffalo (and pile + pertex in general) is very good for active work in the cold (-5 and below imo) where its important to keep moisture off the skin and windchill to a minimum. There not actually that "warm" however. Even a cheap holofill jacket will normally hold more heat and compact down smaller, the buffalo is essentially a pile fleece with a layer of pertex on top, so compresses about as well as a thick-ish fleece. If your standing about in the cold i would really recommend sticking to synthetic fill jackets to keep the heat in. Also remember that if you are active in the cold, and at a comfortable temp in the buffalo top when walking, you'll still need some form of extra insulation for stops. It's a similar idea to being in a sauna or steam room for a while, your body gets used to the heat of it.
Take a look at the montane extreme smock + jacket, i replaced my buffalo when it wore out with the montane smock and its an excellent bit of kit. It comes with a hood that's better than the buffalo offering and a few other differences that i like, no pile at the cuffs or under the arms, better zippers and a zip on hood instead of velcro. Best tried on though, the jackets are cut like a standard jacket, where as the smocks are much more tailored to fit. I'd say that a medium jacket is the equivalent of a large smock. Go outdoors stocks the smocks, but look online for the best price.
Tonyuk