I always carried one thinking I would use it in the event I broke my leg and had to spend the night where I lay, but I question the effectiveness on this, especially with the clothing I carry theses days. It seems the clothing might even outperform the blanket anyways. I already carry a trash bag in both my possibles and kill kit and rarely use those in all actuality.
The space blankets weigh nothing, but there is a little bit of bulk to them, probably better to keep it and maybe even add another.....lol
Rizzy, while our modern day clothing probably is better in the warmth department, a 'blanket', whatever it may be made out of, adds that extra insulating layer to the whole body as one, and will hold in a lot of body warmth, just like a bivy bag greatly increases the effectiveness of a sleeping bag.
I was once holed up in an ambush site many moons ago, we had stomped 40k's overnight, were absolutely beat, then the next night while in the LUP the temperature went south, I actually put my legs into a garbage bag just for that little bit of heat retention, blokes were also wrapping themselves in topo maps that night, how much we would have loved to each have a space blanket.