I told brand name mattresses in a retail store for over a year and as such wanted to chime in on a few the themes people keep bringing up.
This mattress out of the bag, two feet behind me, smells no different than any brand new mattress out of the bad. I don't notice any chemical smell that I didn't with other, non-memory foam mattresses.
All foam compresses from the forces over time. If you use the same spots on the mattress always it will develop sags. I'd recommend not sleeping in the same place every night, and also rotating the mattress 180 degrees every time you change the sheets to make the compression more even.