For those of us who don't wish to carry around a heavy, lunky gri gri (mind you, the gri has a place and purpose and I have used them for it) The Mammut Smart Alpine is my swiss army knife of belayness.
With it's autolocking belay feature, especially in the alpine where rockfall is ever prominent, I feel more secure in my belay.
The autolocking feature is also active on Rappel - this is the device's best feature. While rappelling you are able to go hands free (albeit the manufacturer does not say the device can do this, but it has never failed for me) - I don't do it often, as you never should, but when it is necessary to untangle a rope - it's a nice thing to know you are still secure.
I have also used this device alone to jug up a rope to take photos, and to clean route. I have also used it to jug back up over a roof after missing a rap station.
The device also allows to belay from above up to two climbers. This feature, in my own opinion, is often smoother in it's action than the ATC guide! - although the ATC guide is a great belay device in it's own right.
PROS: It's autolocking feature. It's amazing. It isn't a gri gri, but it not only locks while belaying, but rappelling as well. Lightweight, in comparison to other autolocking devices, and it's versatility are amazing- you can't perform a traditional rappel with a GRI GRi and have the autolock feature- you can with the MSA!
CONS: a bit tricky to figure out at first and requires an HMS biner. But these cons are extremely outweighed by the benefits of the device.