Like OM said they will trap moisture and make your bag wet, but my experience with them is that if the temps are below freezing they'll get a very light coat of frost on them, which doesn't wet your bag and will add warmth. So, really, it depends on conditions.

The inexpensive "foil" "Emergency Blanket" sheets sold at WalMart and Kmart will shred if they get the slightest tear, rip, or puncture. They're maybe good for a one time use, you might get more if you're very careful with them

The "SOL" brand emergency blankets that are orange on one side and shiny on the other are much more durable, and a bit more costly. I use them a lot.

The trick to making either of these work is to have some air space between your skin and the material. Fleece works great for that.

There are some "Breathable" types of heat reflective heat materials. SOL makes a bivy that is breathable and very light. I've not used one yet. Personally, I use a Coleman Fleece sleeping bag inside my 20º bag if the temps get that low. They're fairly light and inexpensive, around $20.

Used as a ground cloth the mylar sheet will help keep your body heat from being sucked up by the ground. It's best to have a layer of leaves or pine needles under it though.

Better yet is a piece of single layer bubble foil insulation for a ground cloth (Reflectix is one of the brand names). Put it under your bag or under or over your sleeping pad. This material is very light, inexpensive, and I highly recommend it.