I can't comment on the Sears mattress or their customer service, but I did recently purchase an Overstock select-a-firmness 11-inch memory foam mattress in soft. My husband and I both love it (we actually got 2 twin xls pushed together to form a King so that we could have them on separate Ergomotion adjustable bases). Get the soft - it is still VERY firm. Probably the firmest bed I have ever slept on, and I like a firm mattress. LOL We've had the mattresses about a month and are very happy. I haven't noticed that it sleeps hot, either, which is a relief. We've had a few hot nights still here (even in SD) and I am still just as comfortable as I was on our old mattress. And I tend to sleep hot.
The only thing I cannot vouch for is it's longevity. It seems like most reviews on the website were positive, but most of them were recently purchased, too. But for me, that was worth the risk because the price was so good and it is what we could afford right now with a one-income family. Maybe in the future when I go back to work we could consider a Tempur-pedic.
The other thing to keep in mind is that Overstock allows no returns on their memory foam products. So, if you get the mattress and hate it, you're stuck with it. The only exception would be flaws in manufacturing or the like, but not just because you don't like the "feel" of it or whatever. A lot of people choose Costco over Overstock for memory foam b/c Costco will allow you to return the mattress to any Costco if you don't like it (their return policy is one of the most generous in the business). Unfortunately for me, the closest Costco is over 400 miles away, so the return policy didn't really do me a whole lot of good. I ended up doing Overstock b/c I could get a better deal and the reviews were so overwhelmingly positive that I decided to take a risk.
But check out Costco, too. Their "Emma" mattress gets a lot of good feedback.