Less is more!
With some air blown in to your platy, wearing thin balaclava or put between your bag and pad, not too inflated overall, you're good to go! They can be slippery, but my ridgerest grips it pretty well. Inflatables may not be as grippy.
It keeps your water from freezing and dual use, so 0 grams.
It could make your head a bit colder, if that is a problem for you. Make sure you have enough air so you are not down to the water, at least in cold temps. Easy night hydration and height adjustment via the tube, but a secure valve is imperative.
I filled one with hot water and fell asleep with it in bed 2 nights ago, so couldn't be THAT uncomfortable..