Just bought one...
I just bought a new mattress about a month ago. GREAT investment.
The bottom line, IMHO, is finding what is comfortable for you. I tried various different memory foam configurations (ala the Temperpedic) and just didn't like the feel. It just wasn't comfortable for me. Also, kind of awkward when you move to reposition, as the foam creates a sort of resistance until you sink into it. (On the other hand, if you're sleeping well, you shouldn't be moving around much.)
I went to the bed stores 3 times. The last time I went, I was there for nearly 3 hours. I would lie down on a bed for at least a few minutes to try it out. The one I ended up getting, I actually took a 20 min pseudo-nap in.
My advice would be to set aside an entire afternoon or morning to do the shopping, so you can really try them out. Bring the same pillow with you from bed to bed and try each of them out. Of course, lie in the position you sleep in.
Also, wear what you normally wear when you're sleeping, so that's the same. For me, I sleep in the nude, so I just threw on my robe and headed to the bed stores....
JUST KIDDING...
Seriously, though, I was pretty sure I had the right bed when I got up off it and had no pain in my SI joints, which almost always hurt when I stand up.
I ended up getting a Kingsdown model. They have a device (it was in the store I went to) - a pressure-sensitive mattress hooked to a computer - that makes measurements of you and determines which of their models would be right. Basically its a matter of how heavy you are. If you're really light, you can go with a pretty fluffy softer model. If you're heavier, you get a firmer mattress. I was pretty dubious, but I tried out all 4 of their standard mattresses and the computer put me on the right one. Although lying on the softest one felt really damn cozy at first, after being there for a while, I could feel my back and SI joints aching.
My mindset was always that the softer the bed the better, but after testing them out, it was pretty apparent that a really soft (or too hard) mattress would have ended up giving me more pain.
Nice thing was, the sales guy used the Kingsdown mattress test info. to point me in the direction of comparable mattresses from other brands. (He uses the Kingsdown system to classify other mattresses - just easier that way.)
Of course he made the valid point that we (Americans) will spend $40,000 on a sports car that we spend 2 hours in each day, but won't spend a few $1000 on a nice mattress that we spend 6-8 hr in a day...
-Randy