Finally some relief!
I was initially skeptical about this mat of sharp plastic. But, having chronically tight shoulders, it was worth a shot! The first 2-3 minutes feel like fire torture: everything hurts and starts to burn. After the initial feeling of being stabbed subsides, the muscles warm and just relax into the mat. I usually lie on it for 30-40 minutes at a time. The mat does leave marks that last for 5-10 minutes, and the muscles are red and warm to the touch--similar to a mild sunburn. However, the tension relief is real! My back and shoulders are noticeably looser after a session on the stabby mat. Yes, it's cheap and has a small learning curve to figure out how to lie down without scraping yourself. If you can handle that, this mat-o-doom may be an affordable path to pain relief!
(It should be noted: I have a decent pain tolerance and use this without clothing. It might be too much for anyone with sensitive skin.)