I did a mini test of the Adventure Medical Kits Heatsheets Emergency Bivvy.
Click here for product info for Adventure Medical Kits Heatsheets Emergency Bivvy 3.5 oz.

I also own the heavy duty version by the same company called Thermolite 2.0 Bivvy. I have not tested this one yet. Click here for information on Adventure Medical Kits Thermolite 2.0 Bivvy 6.5 oz.

The test I conducted was only a "mini" test, but the data is useful.

This test was conducted on my patio.
I have conducted many such tests on my patio with other products down to 4 degrees farhenheit because I get bored in January.
For this test I was wearing a cotton t shirt, cotton socks, and thin cotton pijama bottoms.
Underneath me was a Thermarest Dreamtime sleeping pad, which was awesome.
I put a temperature probe at my chest, one at my knee, and one at my feet. These probes were in the bag and outside of my clothing.

Ok here is the data:
Ambient temperature = 50 degrees
Wind 10 to 15 mph
After a 15 minute test of me laying there motionless:
Temp at chest = 83 degrees
Temp at knee = 77 degrees
Temp at feet = 64 degrees

I was impressed. The bivvy doesn't list a temperature rating, although the heavy duty one from the company is rated at 50f on the packaging. I'll test that one another time.

By the way, I am always a very cold person, and I felt pretty warm during this test, but my feet felt cold.

I have the heatsheets bivvy with me at all times since it is so small.

Let's discuss

Update:
For comparision purposes I did another test.
This test was with a synthetic fill mummy sleeping bag good down to about 25 degrees farenheit.
Here are the results:
Ambient temperature = 50 degrees
After a 15 minute test of me laying there motionless:
Temp at chest = 93 degrees
Temp at knee = 91 degrees
Temp at feet = 83 degrees

I felt very warm and toasty as expected.

For the super small pack size of the Heatsheets Emergency Bivvy I think it's a great tool for if/when the need arises.