We do not have a Tempurpedic, but we have a Spa Solutions (I think that's the brand) from Wal Mart, which is an 8" thick memory foam mattress. I think it was around $250 when we bought it about two years ago. I really like it and my husband bought it thinking it was a really thick memory foam topper (still not sure how he made that mistake, but I'm not complaining). We had also been sleeping on the floor because we gave up on our broken boxspring and concave cheap mattress, which was killing out backs. Someday we do hope to upgrade to a king size Tempurpedic and retire our current mattress to a guest room (in our dream home, in which we will HAVE more than two bedrooms).
I prefer a firm mattress, my husband just has back issues and needs something medium, and the memory foam has been pretty good. The only other consideration is getting a decent platform frame. I wish we had one that had a solid base rather than the wood slats, which tend to pop out on ours (especially if you actually do anything other than sleep on your bed). We bought a cheap captain's platform frame with drawers in the base but I think I could modify it with plywood if I'm ever really motivated.
So, if the cheap one is pretty good, then I'm sure the Tempurpedic would be a great investment. If you can afford it, go for it. There is NOTHING worse than sleeping on a cheap mattress for years.