The mattress is definitely more like 3-1/2 inches thick. It is dense, but I added a foam pad and mattress pad to get a little more comfort. It is for my 4 year old, so being firm isn't that big of a deal. I only had an 8" clearance to work with under the bunk set, so this worked out perfectly with the low-profile roll out trundle frame I purchased with it.
I purchased this mattress for a trundle bed to be used when my daughter comes home to visit from college. Many mattresses that say they will fit a trundle really do not as they are too thick (I have many snagged and ripped sheets to prove this!) so I was specifically looking for one that was thinner. The mattress is probably closer to 4 inches after expanding but is not "thin" feeling at all. My daughter and another college-age friend both used it while home on spring break and both said it was very comfortable. It's also lightweight so would be easy to move and throw down on the floor for kids over watching TV, etc. Overall, a very good value.
I usually just sleep on a plywood platform with some extra blankets for padding so I'm always concerned that a "real mattress" will be too soft. I've slept okay on some high end firm mattresses but didn't want to spend that much. This has turned out to be perfect for me. I don't know if most people would find it too firm, but if you're used to a futon that should be okay.
I was looking for a trundle mattress for a Land of Nod bed in my son's room. The size is smaller than a typical twin mattress. Land of Nod wanted **** for the custom mattress that they sell. I couldn't justify that for a occasionally used kids guest bed! I ordered this and it fits snugly into the trundle and is very comfortable. I definitely recommend this one!