Having managed with a standard belay device for the best part of 30 years and avoided 'Gris Gris' type devices. I am totally pleased with Mammut Smart 2. Have used it to arrest falls, top roping and leader belay. No problems at all. It is very safe with minimal slippage and is safe in the hands of a learner. The default is to attest the rope which it does very well. I'm not sure how it would manage descending, I will probably stick to my belay plate or figure of 8. Highly recommended.
I bought this after my climbing partner took quite a big fall. I didn't drop him or anything but it was just one of those things. I thought I'd give a new device a go instead of my ATC and thought I'd try this out of the assisted braking ATC style devices. A Gri Gri or other similar devices just seemed like a bulky faff. I really like it. I find paying out to be super easy and lowering too. It is definitely less tiring than holding someone whose hanging on to a draw whilst resting with an ATC which is nice too. The catch from a fall is pretty sudden on the 9.8mm rope I'm using at the moment so I am trying to improve my dynamic belaying too which is fun. Overall, feels like a decent bit of kit.