We've used one since '07! Goes under the sheet between you and the mattress (as others have said) and works great! I have two group 29's and we have never had a issue with the power draw. We have used down to -6f OAT with the furnace set to around 55. Batteries held up fine. An electric blanket on top just will not do anything for between you and the mattress, so you're still uncomfortably cold at the point of contact with the mattress. You don't really need one to cover the entire mattress, just the main torso and legs, you're noggin is on a pillow (and probably hollow anyway). I think we paid around $70-80 but it's been well worth it.