I don't think it's a good reason to bring the mylar/bubble wrap is for insulation value or padding. It won't provide much of either. But it may be good to bring it for its ability to reflect your radiant body heat back at you.
This is easy to test to see if it's effective. Get a one foot square piece of it and take it outside on a cold day and set it on the ground and sit on it for one minute. Then sit on the ground without it for one minute. You can feel your body heat radiate back at you almost instantly. I've done this and I'm convinced it works great for a butt warmer when sitting on the cold ground.
I haven't tested it under my entire bag yet, but I'm going to. You can probably accomplish much the same thing with one of those mylar "Emergency Blankets" under your bag and on top of your pad. The plastic bubble/mylar laminate does provide some durability and makes the product more functional with little cost in weight. That's why I'll start working with it first. But the Emergency Blankets are cheap, so I'll play with them too.
I'll let you know what I find out <img src="/forums/images/graemlins/wink.gif" alt="" />
"You want to go where?"