It's one month since I use it and I'm really confident with it. It works really well. I have had some falls since then and it has always braked really firmly.
I read, and it can also be felt, that its braking is dynamic meanwhile the Grigri is static, so I feel secure when I use it not only in sportive climbing but also on classic routes. In this kind of routes, friends, cams and rest of anchors suffer in falls with high shock loads, and there is where the dynamism of the Sum makes climbing safer.
Moreover, with the Sum I use my new thin ropes without problems, which I couldn't do before.
In my opinion the best belay descender ever.