We have a 3" topper we bought at Target. We put the normal fitted mattress pad over it to help keep it in place.
It definitely makes bed feel a little warmer, but that's OK for now - we save a little $$ on heating.
At first, it felt like I was sinking into a giant hole every night, but after about a week, I started to REALLY love it. We've got flannel sheets on the bed right now and we call the bed the "Flannel Smooshy." SOOOOOO comfy. Very supportive. A good way of renewing our aging mattress without having to replace the whole shebang.