I have the Owlet Smart Sock for my twins. A snug fit has kept the sock in place every night for the last eight months. I've had the occasional Base Station/WiFi disconnect notifications, but the inconvenience of a few technical difficulties are nothing compared to the relief of being notified when necessary. The Owlet has helped me with BOTH of my twins.
To me, the Owlet Smart Sock is priceless. I am so thankful for this device, and I'm even more thankful that I took the time to really learn about the product, talk to customer service, and read the reviews so that I understood how to best use this technology.