The reflectivity as related to personal warmth isn't the most important factor. The biggest factors are that the space blanket stops both evaporative and convective heat loss. It offers this function in the smallest, lightest, most "durable" package available. If you are truly in a survival situation, then the small amount of IR reflected back to you just might make a difference... but there are other benefits to the metallized surface.
Ever seen a reflector oven? Aside from personal protection, a space blanket can be rigged as a mini-lean-to in front of a fire, reflecting that heat back on you. Shoot, the reflective surface could even be used for a bit of signaling. The space blanket is the smallest, lightest... and cheapest!... emergency shelter you can find.
Having been a W-EMT, ski patroller, and SAR member, I have wrapped people in space blankets. They have reported feeling noticeably warmer. (In case you're wondering, I have no great life-or-death stories involving space blankets... the wonders of ATVs, ambulances and other fun.) If you're hung up on the efficacy of the IR reflection, just think of it as a really handy, versatile VBL. (And we know VBLs can add 10 or 15*F of warmth. If you're hypothermic, every extra degree of warmth can help!)
Over the years I usually just carried one (or two!) in my pack. Then I realized that every time I've been remotely close to getting myself into a survival, lost in the woods type situation I'm away from my pack. I went down a trail spur, or "just over to that clearing," or some such nonsense. Now my survival kit, including a flat-folded emergency blanket, has permanent residence in a pant pocket.
In short, I think space blankets are remarkably light, compact, cheap insurance policies. They do make a difference! Especially when you've got nothing else. And they're small enough and light enough that you can easily have one on your person.