First off the mattress actually reached 10 inches.  I would definitely say to give it at least the full 48 hours.  It's firm but just right.  If you've stayed in a Hilton before it very much reminds you of that feeling.  I did get a little hot recently which Memory Foam mattress are infamous for sleeping hot.  Nothing a ceiling fan can't solve.  

This is definitely not something that will eliminate the need for a box spring.  As a matter of fact I would definitely suggest the box spring as I suspect that probably adds in being able to balance and support the body weight thus keeping the mattress from getting impressions in it.  I picked up an 8 inch Box Spring from Big Lots for 100 bucks!