I was recently reading Kevin Mitnick’s “The Art of Invisibility” and found that he’d also recommended these devices. I’ve been using them for several years as it was always unnerving to plug in a mobile device into a work computer to recharge only to see that there was a request to mount them. Additionally, my laptop would occasionally want to tether data via my cell phone. In an effort to block data transfer and leakage, something was required. These simple and cheap devices allow for power but no data to be transferred via the USB port.
WARNING: there’s always the possibility that any USB device could be compromised, including these… keep them in sight and under your control at all times.