These pants are awesome. So awesome, in fact, that I wear them despite the fact that they're a bit small. Like other reviewers mentioned, they run about a size smaller due to the thickness of the fabric. They're incredibly soft and plush. And warm! They do tend to collect lint/pet fur easily, so be forewarned if that's an issue for you.
For size purposes, I'm a 5'6, 135lb woman with an athletic build, and a 27in/sz 4 waist. I bought a size XS (only XS or XXL available at the time) and as I mentioned above, they fit fine in the waist, but are tight in the legs. They fit more like leggings than lounge pants, and I'd definitely rather they fit like pants! The waist sits lower than I'd prefer, again, I think because of the thickness of the fabric. A simple redesign will take these from almost perfect to perfect.