I asked this question a few months ago. We found a memory foam mattress on Walmart.com and had a king sized, 13" mattress shipped to the store and it fit in the back of the SUV. (They fold it in half and roll it up.) We got it for $630 after taxes! We thought, for that price it was worth trying. SO GLAD WE DID!
LOVE IT! I did find one review that said it was better than the Tempurpedic mattress and yet so much more affordable. I can't compare exactly, but it is awesome. It did "stink" for a few days, but we bought it back before we could open our windows (too cold out). The smell is normal and went away pretty quickly. We didn't jump on it or knead it like you are supposed to, and it was fine. We've had the 3 year old jump on it, though!
We put ours over our other box springs, but will consider getting rid of those when we move and just use the mattress on a platform bed. Very versatile. (We put our old mattress on Craigslist for free and it was gone in a day).
Our 3 1/2 year old wouldn't sleep in her own bed after she slept in our bed a few nights. We are moving and will get her a memory foam mattress when we get to our new house.
Thought I'd give you another option. :)