I’ve had a these retrofitted to two skinsuits, less for the supposed 10W aero benefit and more for the avoidance of pin holes in expensive skinsuits, which inevitably shorten the garment’s lifespan. I rode my club skinsuit in a club event, and had my partner put the number in while I was wearing the skinsuit. This was very difficult, and you certainly wouldn’t want to ask someone you’re not intimate with to do this! On the second occasion I was using a Castelli Bodypaint skinsuit, and pushed the number in before putting the skinsuit on. Even that was a bit of fiddle, but mostly because my hands were cold. In use, the number pockets are great - none of that pricking fingers when putting a number on, or pins coming loose while warming up or racing. While the number is held really snug and with no flapping, I’ve no idea if there’s really a discernible aero benefit, but at least the days of pinholes in cycle clothing are receding!