Ah, the Woobie! Beloved (possibly!) kit of many USA GIs from Vietnam onwards hence the USGI tag. Basically, it is a poncho liner designed to fasten to a military poncho to give extra warmth but its far more versatile than that. With a poncho and liner you can:
1. Have insulated rain gear on cooler days.
2. Turn the two into a waterproof, windproof bivvy bag. How comfortable you’d be really depends on how “hot” you sleep but I reckon about 7C is the lowest for me. Although the combo isn’t breathable the Woobie is synthetic so keeps most of its warmth when wet and dries quickly.
3. A separate tarp and warm weather sleeping bag – say 10-12C is about as low as I’d want to use it. Or open it out and use it as a quilt.
4. A mat to sit on, doze on, picnic on, etc.
5. Use the Woobie as extra insulation at lunch stops or in the evening in camp.
6. At home when dozing on the settee.
7. Keep warm while sitting in a car while waiting for a breakdown truck.
8. A bedspread when sleeping in dubious hotels.
9. House camping at a friend’s place.
No doubt there are lots of other uses too when you need a bit of extra warmth.
Positives: tough, quick drying, multi-use, cheap at about £20 for a new, unissued one, once you’ve had one you’ll find it something you’ll use a lot.
Negatives: fairly bulky as synthetic, at 650g the warm:weight ratio isn’t outstanding, not fireproof, camouflage is the colour.
A poncho and liner will set you back less than £40 for new kit. For that you get a versatile outfit for warmer weather camping which is pretty good for people wanting to try out tarp camping without spending a lot of money. Or trying the quilt system rather than a normal sleeping bag and/or boosting the performance of an existing sleeping bag. If you’re unlucky and injure yourself (or become benighted) when out you’ve got a simple to use shelter or bivvy bag. Being military, it is robust but there is a weight penalty but it is not back breaking; my Woobie and German Army Poncho come in at just under a kilogram or 2.2lbs.
Within its limitations, the Woobie is worth serious consideration. I like mine a lot.