I could not sleep for more then 3 hours a night.  At first I thought they had wood or rocks in there, but if you press really, really hard on a particular spot you can clearly get the mattress to yield.  Sort of a similar feel to a baby's crib mattress (they make em really firm these days to prevent SIDS) but even firmer then that.
 
Just out of curiousity, anyone know what these are made of?  Something cheap and highly durable presumably.  
 
Ok, not all curiousity, I might want to buy my worst enemy one as a gift.