Mr Park queried: "I've never had to use one and I hope to never have to use one but are the thermal emergency blankets functional at all?"
You should try one before you need it!
The mylar ones tear easily, but they will cut the wind and reflect heat. The Adventure Medical blankets are tougher, and more expensive, heavier and a bit larger. I put one in a Seal a Meal bag and sucked the air out, reducing the size by about 50% and the Seal a Meal bag is much tougher.
They can be used like a blanket, or placed behind you as you face a fire. I think they have good worth as a shelter roof– build a simple lean-to, add brush for a roof, add a space blanket and then another layer of brush for a waterproof shelter. A large garbage bag will do about the same for shelter-building.
The Thermolite bivies are about as good as any bivy on the short term, but not breathable, so the foot vent has real purpose.
If you have an extra layer of clothing like you're SUPPOSED TO, the bivy sack style space blanket or the Thermolite bivy will get you through a long night. You will wish for your down sleeping bag, but they will keep the wind off you.
I think they make a good paring with a poncho shelter. The dawn will find you tired. grumpy and cold— but alive.
If you are stuck out below treeline, I think you can forget about leave no trace principles and build yourself a nice fire and insulate yourself with boughs. You just need that nice Swiss Army knife with a saw and some sort of fire starter.
If it is in the dead of Summer and fire danger is high, it gets iffy— like if you don't you might be really cold, but if you do, you might get more attention than you want after burning down a few hundred acres of forest. Use your head!