I've had this one for almost 2 years
Highly recommend. I was a little skeptical since the price is insanely good, but it was absolutely worth the money. I'll probably never go back to a regular spring mattress.
http://www.amazon.com/gp/product/B003CT37KQ/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
I've had this one for almost 2 years. My wife had a lot of back issues before we got it. Love it. I hate staying anywhere else, because I sleep a million times better on that mattress. I'm 190 and always sleep in the same spot. Absolutely no divot at all. It is a little hard (don't jump on it), but molds itself to you and softens from your body heat. It comes in a tiny box. It was really weird to open that tiny heavy box, cut the vacuumed plastic, and watch a huge mattress grow.
The hinge mechanism comes from: http://www.sgs-engineering.com/gas-struts/bed-hinge-mechanism
I purchased their "large bed" mechanism. When I purchased the mechanism, I contacted their support and told them the approximate weight of the top part of my bed. I guesstimated the weight of the wood and the mattress weight is listed on Amazon.
If you don't contact them the SGS site asks the size of mattress that you will be using and will send something known to work.
My bed is US "Full" and I used this mattress: https://www.amazon.com/gp/product/B003CT37KQ/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
EDIT: There was an element of "winging it" when I purchased the mechanism. My back of the envelope math required each strut to be able to support 90LBS. This is what I asked for. Works very well for me. The bed is easy to open and stays opened. Requires not too much force to close it.