I used to sell mattresses and back then, the better high-end mattresses were *supposed* to last 15 years. They had a guarantee for 15 years. However, I've never had a mattress that lasted that long. We went 12 years once. In my parents house, the guest room bed was so uncomfortable, I bought one of those egg crate foam pads and put it underneath the mattress pad and that helped a WHOLE LOT. You might try that--it's a fairly cheap solution (less than $25--look at WalMart)
Another thing- do you turn your mattress? The mattress manufacturers usually recommend turning every month and I don't know many people who actually do that. Of course, some pillow top styles are only pillow top on one side...so you can't turn them over. You can turn them head to toe, though, and that might help.
As for different mattress styles- we stayed at a B&B one time for THREE NIGHTS on a sleep number bed and I will NEVER sleep one of them again. I know some people really like them...but they are not for us.
My son has been on a mission internship in New Orleans and he had the opportunity to sleep on a tempurapedic (Spelling?) and he LOVED it...felt it was the most comfortable ever, except it was hot and this place didn't have air conditioning.
Good luck!