Here in the east, I've found that midweight is pretty much an all-purpose, 4-season base layer. They're a great layer to put on in camp, when it's cooler, and nice to wear in the sleeping bag. In cold weather, they're good for hiking under a pair of shorts; they keep my legs warm enough, but let the excess heat built up by activity dissipate. (I don't like hiking in long pants, myself, but the long-johns' close fit keeps them from feeling like long pants.)
I once bought a set of expedition-weight wool longjohns, and I think I wore them once. The problem is that, by the time it's cold enough to need them, it's too cold for them alone. They're definitely too hot to hike in. In camp, they're usually not warm enough by themselves. If I add a pair of light down pants, I get too hot. So, it's back to the midweights, which are just right under the down pants. (Layering the midweights and expedition weights just means too much bulk and the constricting feel, I think, cuts off circulation in my lower legs, which actually makes me feel colder!)
Stick with mid-weights.
Edited by Glenn (04/23/10 07:13 AM)