User Review: Helly Hansen Fly Pant
For both the crew top and fly pants, I prefer the HH Lifa to other baselayers because it is polypropylene and not polyester. My most extreme use for it is playing cold weather ultimate - continuous running in nearly freezing weather and wind and rain.
It seems to keep me warmer and drier than the Patagonia or Underarmour equivalents I've tried. I also use it as a baselayer for wading while fly fishing, and as an extra layer to carry for hiking. It is lightweight and packs tightly, so it is a good combination if you might encounter unexpected cold wind or cold.