Gri-Gris have been a common sight at local crags for many years. The "auto-locking" characteristic of the device offers an excellent back-up to the belayer's brake-hand and also takes some pressure off of the belayer during hang-dogging sessions. A few years ago, Trango released a capable competitor to the Gri-Gri, called the Cinch. One of the few drawbacks of these devices is the propensity to lock-up just as the belayer is trying to quickly give out slack during a lead belay. In many instances, belayers feel forced to let go of the brake-end of the rope in order to unlock the device and feed out rope.
Both Petzl and Trango recently released revised instructions for belaying a lead climber with the devices. Both techniques greatly enhance the effectiveness of a lead belay, making the devices even more valuable as primary belay tools.
To download more detailed instructions from each manufacturer, click: Petzl & Trango