Overall, I think this is a good value for the price, but you do get what you pay for.

This mattress is closer to 4 inches, not 5. When you sit, you can definitely feel the hardness of the platform underneath, but not when you lay on it. It is relatively firm (which reviews noted, and part of the reason I bought it). 

I found it extremely comfortable to sleep on at first, but after a couple of nights my body left an indent in the mattress that doesn't go away. So now, it kind of slopes down. I weigh about 200 pounds, so someone smaller or a child may not have the same problem.

I've been sleeping on the other part of the mattress for a few nights. Not sure if that will help. 

This mattress is on our guest room bed, where I sleep most nights. I'll probably flip it when I have a guest come stay. For me, this was an inexpensive practical alternative to a more expensive mattress, but I think it would work best for a smaller or lighter-weight person.