Note the difference between an emergency Space Blanket and the Sportsman's Blanket. Both have good and bad characteristics.
Space Blanket
GOOD: Very lightweight (couple ounces). Compact & easily stuffed in a pocket or corner of pack, at least while still in their original wrap-job. Due to reflective surface, I suspect it's a bit better than a trash bag for keeping you warm.
BAD: Fragile, basically a "use it once & throw it away" item. These are VERY susceptible to holes getting poked in them easily or snags tearing the material. Once an intrusion (hole or snag) is introduced, the material seems to continue tearing apart further. No edge taping or grommets.
Sportsman's Blanket
GOOD: More durable than Space Blanket due to multi-layer laminated construction, including the reinforcing scrim layer in the middle of the stack. Has sewn-on tape edging and grommets that facilitate lashing the blanket in place.
BAD: Much bulkier and heavier to carry than a space blanket. They are big enough to need a daypack-sized carrier to haul it around with the rest of your kit. If you wear a hunting vest, you could stuff it in the game pocket in back. Sportsman's Blankets also get a bit stiff in colder weather.
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In spite of their increased size & weight, the **ONLY** one of these that I recommend any more is the heavier Sportsman's Blanket. IMHO the lighter weight space blankets are just too flimsy for serious survival situations.
For bright orange Sportsman's Blankets at a good price, check out Emergency Preparedness in Seattle:
http://www.emprep.com/all weather blanket.html
Otherwise, for the red or OD ones I get 'em at Cabela's:
http://www.cabelas.com and search for "space blanket"
Blue colored ones are carried by Campmor, but they are 20% more expensive than Cabela's or EmPrep links above:
http://www.campmor.com and search for "sportsmans blanket"