We just LOVE our memory foam mattress and we are having no problems at all after 18 months.
We put our mattress in a zip up bag designed to stop mites. It also stops the chemicals from getting into the bedroom.
Our mattress is exactly like the 12 inch thick Queen Size $3,500 Tempurpedic, except it was only $400 including shipping from Amazon.Com. Amazon has simlar models this year but are somewhat more expensive.
We also put a fabric topper on it to absorb any sweat.