I used this when camping on cold nights in the winter. It's actually really good for that purpose. It DOES add warmth. I did no scientific test, but after each night when I woke up, the sleeping mat was warm (from my body heat, being reflected down, then up), and the tent floor was cold. That means it does its job with reflecting warmth.
It's extremely lightweight (less than 100g), and very portable and compact.
However, it's not very durable. After only using it for a few days, the seam for one of the edges tore up. I used the mat normally, yet this happened. It can't be very durable for this reason. The seller kindly offered a partial refund because of this.
Most likely, if you use this product, you'd need to repair it yourself several times. But it's pretty easy to repair with eg duck tape. Therefore, I do recommend it still.