The quick answer is it depends. It depends on what you intend to use them for, and this is critical. If you are looking for a mattress topper because you have an old mattress with little support left. If you are looking for a topper because you have heard good things about memory foam mattresses, the answer to that question is no. But why? While memory foam is very comfortable, and offers a great night's sleep, it can't do it all by itself. The reason memory foam mattresses are gaining in popularity is because of the system, not just the memory foam on top. The major memory foam bed makers have anywhere from 1 to 4 inches of memory foam as the top layer. Under this layer is the support structure for the bed. Most use more foam, just a different kind, and mostly latex for the support in the mattress. And that is specifically why the answer is no. If you have an older bed, which is no longer comfortable, putting a layer of foam on top will not correct the support issues of the mattress. Of course it will be softer, but it will rarely solve the problems you are attempting to solve with the topper.
So when is a topper a good idea? OK, we all want to save some money right? And that is the primary driver for the topper sales, extending the mattress to avoid buying a new one. That is the wrong way of thinking of it. If you are looking to save some money with a topper, and get a good night's sleep, consider buying the topper when you buy a mattress. We hear the question now... How does that save money?
Let's use our store as an example. We sell a very nice air technology bed with 4 inches of memory foam as the comfort materials. A queen set is $2999. We also sell an entry level bed with a couple inches of latex comfort material. A queen set is $1150. That's a wide gap. Let me just note that getting the memory foam option from the factory will provide a higher quality memory foam than is generally available, especially at discount stores. That being said, for a couple hundred dollars you can get a 2 inch topper, and it will work well with the lower priced bed, just not as long, and not as conveniently. The big point is that you have saved money over the more expensive bed, and have great support for the topper.
To be completely honest, the topper will not last as long. You may have to replace that topper every 5 years. Either of the beds mentioned come with a 25 year limited warranty, and can last for much longer than that. 25 divided by 5 is 5, so the total cost of going the topper route is going to be around $1000 over 25 years. if you add that to the initial cost of the bed, it nears the price of the more expensive bed, especially considering the foam in that bed is twice as thick, and a higher quality foam, and generally a bit more dense.
It's up to the customer in our store. We want to sell the support end any way. With the proper support, you may not even need the topper to begin with. And our entry level beds in queen start around $500, so just a few hundred dollars more than the topper alone. The support of the bed is what eliminates, or at least minimizes, aches and pains. It's the support that helps our customers sleep all night and awake refreshed. It is the support that will limit tossing and turning. It is the support that is important in a new mattress, and the first thing to go in spring beds. That is why they want you to replace every 8, while we say this may be the last mattress you will HAVE to buy.
At any rate, that is our take on mattress toppers, it depends on where you are financially as to whether it makes sense for you. But again, toppers will not fix a broken support bed.
We hope that helps...