The use of sleeping pads like this one have been combat proven for decades. While those who have never slept on one will comment about how thin they are, they are thick enough to provide the layer of support necessary to ensure a restful sleep on the ground. With the padding provided by these mats, your back will not be unsupported between your sleeping system and the ground.
These GI Foam Sleeping Pads are made by the same factory that produces them for the military. They are designed to be easily rolled up and tied to your backpack, and are quick to unroll and place under your sleeping bag. The size is big enough that you will be fully supported while sleeping on the mat with no problems. With the closed cell design of the foam mat, there should be water resistant performance to prevent any soaking in the rain.