The increase in popularity of memory foam mattresses is due largely to two main factors; the pressure-relieving qualities and the fact they are now more affordable than ever. There are many companies now selling a range of memory foam mattress models, but they're not all made equal.
There are a few main things to consider when looking for this type of mattress. The density of memory foam used in the mattress, the softness, and the depth. The density is arguably the most important as it's this that provides the ability for the mattress to support the contours of your body.
Many cheap memory foam mattresses will try and sell the depth of the mattress rather than the density, but make sure there's enough memory foam in the mattress to make it a worthwhile purchase. Models such as those from the Tempurpedic mattress range have a high amount of memory foam in their mattresses, which is part of the reason why they are priced at the higher end of the market.
Many memory foam mattress buying guides bring up the problem of heat build-up. This is often a complaint with some mattresses, but there are materials such as coolmax which help keep the user cool during the night. You should also look out for cheap memory foam mattress imitations. They are cheap for a reason, and it's often because the memory foam is poor quality, or there isn't much of the memory foam material in the mattress. One of the main reasons for buying a memory foam mattress is to get a more comfortable night's sleep and relieve the pressure placed on the neck, spine, back and joints during the night, so I'd recommend not buying the cheapest you can find but rather the best deal for your money.
Memory foam mattresses are suitable with most bed bases. So long as the base is even, the memory foam mattress is what provides the benefits. If you have a wooden slatted bed frame though, try to space the slats no wider than a couple of inches apart.
All mattresses come with a warranty, but they do differ in length. The cheap mattresses may only have a 12-month warranty, which isn't long in relation to the length of time a mattress should last for.
If you don't have the budget or the need to buy a memory foam mattress, but still want the benefits, then a memory foam mattress topper is a good alternative. These can be bought at different depths, but I'd recommend buying at least a 3-4 inch topper, else you limit the effects of memory foam with anything thinner.