I made some cushions for forklift operators with this find of foam earlier this year.
Some observations, it crushes to flat pretty easy.
Like you would need 3 or 4" (probably more) of padding before you're weight against it won't squish it flat.
Constant pressure points like your lower back area, that area of the pad will eventually stop springing back to full shape.
My experience says you probably won't be happy with it as a seat you're going to spend any time in.

They do make some killer gel padding but it was too expensive for what we were doing.

Disregard the link as to the supplier, I'm sure you can find something similar on ebay.
http://www.alibaba.com/product-gs/599457903/Memory_foam_Cool_Gel_Seat_cushion.html