I can now write a proper review of these now I have used them at my last music festival.
Does what it's supposed to, you put the tinfoil thing on and then just wait for a minute till you heat up.
I used and re-used one blanket throughout the weekend. They do become rather crumpled and noisy but remain warm.
After repeated use I think a very crumpled blanket starts to smell a little tinny after I've had it lying around for a while, though this is not a major issue or very noticable.
Doesn't matter if you wear a coat or tshirt under these, though I recommend covering your arms as you can imagine the plastic coating will stick to you slightly.
I bought mine from EVAQ8 for £4.80 total, which at 96p per blanket is a good price, average internet price appears to be around £1.50 per blanket. They are each individually wrapped which is useful for packing them away.
The blankets are sized at 2.15m x 1.5m which provides full coverage.
(Poundland sells two 2m x 1m blankets for £1 & I also tried these, though one of these is only enough to cover shoulders, so that ends up a worse deal.)
I think the warming effect of most emergency blankets is the same, but a bigger size is better. I have not tested an 'extra thick' space blanket -I'm not sure if that would crumple less but I don't imagine the warming effect being much different.
Can use these as floor insulation in tents too if you don't have a foil-lined rollmatt.
I might go for an actual foil-lined camping blanket after this.