I have always been a bit leery of "automatic" belay devices, given the way I have seen them used by less experienced belayers and particularly at gyms. At the recent Outdoor Retailer Show, I took the opportunity to talk at length to the makers of several Grigri-type devices. One in particular seemed to me to be an improvement in several ways over the original Grigri (Petzl's device by that name). A large part of my discomfort has been that belayers using the devices seem to have a tendency to pay less attention to what the climber is doing and to be dependent on the "automatic" locking of the devices. With a tube-type device, the belayer has to pay constant attention to the climber and to his belaying.
The manufacturer (a competitor of Petzl's) pointed out that there is a strong statement on their device to the effect that the device is for "Expert use only!" He was adamant that it, and similar devices in his opinion, is NOT for use by beginners. When I asked for further explanation, he said that there is evidence that a larger fraction of auto-belay devices drop their climbers than of tube-type devices. I later went to several other makers of such devices and got similar comments.
Part of the reason is that less experienced people have a lot of difficulty regulating the speed of lowering the climber after a climb. Another contributor is that the complacency induced by the "automatic" locking leads to having the climber have excessive slack. The several manufacturers I talked to noted several other modes in which the climber could be dropped or lowered too rapidly.
Yes, I use Grigris and several other autolocking devices (including the Kong Gigi, Petzl Reverso, Trango Cinch, and others), particularly on walls. And I have used them on solo climbs (the Silent Partner is more suitable, though). They have their place. But they are not (per several manufacturers' comments) a panacea. Like any other piece of climbing gear, experience is needed. Watch for the "Expert Only" note to appear on more of these devices.