I have used the Grabber warmers, and had reasonably good success if they were fairly new, sometimes the ones in the discount stores have been around for a while. I have used the small hand warmers for inside gloves, and the adhesive toe warmers inside my boots, and also when sleeping. I find that if I put a thin liner sock on, and stick a toe warmer on each foot, then put a second heavy wool sock over that, my feet stay comfortable all night. (You have to have everything else covered with a decent layer too. Your feet and hands will start to chill if your body starts getting cold anywhere.)

Two weeks ago when I was camping I was using a -10 degree bag, and a full length inflatable Thermarest Prolite 4. I used the toe warmer/dual sock method both nights. Temps were in the high single digits, winds ranged from 10-25mph, so it was cold, below zero wind chill all weekend. I slept like a baby, except for the ice/frost that would form near my nose and mouth on the balaclava I was wearing. I would sometimes wake up with cold crusty stuff against my face when I would roll over.

I have not tried these yet, but, Grabber also makes larger warmers called "sleeping bag warmers". Here's a link:
http://www.warmers.com/ItemDetails.aspx?itemid=SBWP

Does anyone have experience with those? I like the toe warmers inside two layers of socks, because they follow my feet when I move around.