I've laid on an actual Tempurpedic, and it doesn't feel as nice or high quality as the NovaForm memory foam mattress from Costco.com. It's too soft and plushy. It also depends on which Tempurpedic, because I believe they have different models with different quality of foam. I've laid in one where it felt cheap, and another which felt almost as good as mine (both were Tempurpedic brand). I really had no clue what I was going to get when I purchased the beds from Costco.com, especially since Costco never has any display model to feel at the actual store. I have a bad back and I never wake up with a sore back anymore. The best deal is when you have the $50 off coupons and it comes around every 3 or 4 months.
Here's a link to the NovaForm. If it doesn't work out for you, you can't beat Costco's return policy (minus their new electronics return policy)
Costco search results for "memory foam"