I was looking for a layering piece, and I think this is it!  The test:  4 hrs of golf/walking in 50-degree weather under a regular skirt.

Pros:
  - Very thin, but not see-through, not too shiney, and very comfortable - forget that I'm even wearing it.
  - Even by itself, warm but not hot for 50-degrees.  I imagine it will be fine for 40-degree weather, too.

Cons:
  - After 1 hr of wear under a skirt, they started slipping down (I'm 5'4", 115 - 120 lbs, usually wear an xs and ordered an xs).
  - After 2 hrs, the seam in the waist started to irritate my skin.

The v-shaped waistband is made of the same material as the leggings - it doesn't cut into my waist, but it doesn't hold up the pants if you wear it on it's own - may be a non-issue if worn under pants (haven't tried that, yet).  Also, the seam that connects the waistband to the pants is not a flat seam, so over time, it becomes irritating.

Overall, I think the cons are very minor.  (Expect to wear it under dress pants and jeans when the weather gets colder - maybe will tuck a cami between me and the waistband to prevent irritation.