ScottsSoHot saidIt does form to your body, but I like moving through the night, every time you shift it re-molds to you and isn't comfy like my wool topped mattress.
That's was my experience, which you may not appreciate during a quick sampling at a mattress showroom. The longer you lay still the more it conforms to your shape, making a body mold of you.
But the foam doesn't bounce right back when you move, hence the common term "memory foam." I found it was like sleeping in a trench, every time I tried to role or move. I thought it a very claustrophobic sensation.
On the other hand, a spring mattress hurts my back. I do best with an air mattress, having had great success with a number of Select Comfort/Sleep Number models over nearly the last 20 years. Best mattresses I've ever used, and I've had them in various sizes for different bedrooms.
But as others here have noted, neither an air nor foam mattress gives you the "bouncy-bouncy" of an innerspring. They're surprisingly dead, if you're accustomed to humping away on a conventional spring mattress. But wonderful for a bad back like mine.
BTW, a mattress of conventional foam will tend to develop depressions over time as the foam permanently compresses, mimicking the spring sag of a traditional mattress. But an air mattress can never sag, unless you get a model with a thick pillow top with standard stuffing, and that additional material is subject to becoming compressed. That's why I always buy mine with a rather plain flat top (which is also cheaper), or at most a thin pillow-top effect. My back does best with a totally flat sleeping surface, usually at a fairly high air pressure setting, another nice feature you have with an air mattress.