An assisted braking device, operated within the fundamental principles of belaying, is an especially valuable tool if climbing teams prioritize backups. If a belayer takes an honest self-assessment of all the things that might thwart the best intentions of a diligent and competent belay, then it is difficult to justify not prioritizing backups. It is perfectly reasonable, and perfectly human, to accept that any number of sights, sounds, and distractions compete for a belayer’s attention. (American Alpine Club)

Whichever device you buy, it’s critically important to learn the proper techniques for each device—they vary widely. If you don’t get these right, you not only won’t be able to feed slack quickly to a desperate leader, you also may endanger your partner. [See sidebar on next page.] Because of this learning curve, we recommend you choose one assisted-braking device and stick with it. And, of course, no device is completely foolproof— the most important aspect of belaying with any device is paying close attention to your partner. (Climbing.com)

The GRIGRI has long been the go to assisted braking device for years, being the original and, up until quite recently, one of the few products in the category. More recently this has changed, and the GRIGRI has more competition than ever to retain its top spot. (UK Climbing)