Russell M.wrote a review of Mammut Smart Belay Device on July 1, 2013
Familiarity: I've put it through the wringer
I got this belay device for lead belays, either in the gym or for single-pitch outside. It's worked great for me- it catches falls smoothly and easily, and works great at paying out slack for clipping.
Lowering the climber took a bit of practice to get smooth, but works well for me now. I definitely recommend watching the instructional video on the Mammut site. I wasn't sure if I should keep my thumb on the hook at all times or only while giving slack. Now I permanently keep my thumb on the hook for a smoother and safe belay.
As a lefty, I don't like lead-belaying with a grigri, since it is designed for your right-hand being the brake hand while rapidly paying out the slack needed for clipping. I'd either short-rope the climber during clipping, or be tempted to take my left brake hand off to help feed the rope quicker. The Smart is symmetric, so I can use it the same as a righty. I never take my brake-hand totally off the rope, but the lock-assist means that it takes hardly any effort to keep the rope locked off, which is quite handy when the climber is hangdogging a project. My girlfriend, a righty, also likes the belay action, and usually uses the Smart instead of her own grigri.
However, I don't often use this device while toproping in the gym. Since the toprope ropes are a good bit thicker than the lead ropes, belaying on those ropes is sluggish, with more effort necessary to take up the slack as the climber climbs. For this, I usually just use my standard ATC.
While the alpine version needs a HMS carabiner, for the single-track smart device, a NORMAL CARABINER works much better! The HMS beener had the nasty habit of cross-loading while in use with the Smart.