I bought this after I got a really good demo on a climbing trip. It feeds real easy if you use a rope 10mm or less. It is a bit heavy but I only use it from the ground and never bring it up on the climb. Compared to the gri-gri and the cinch this is the device I prefer. It takes some time to learn how to lower smooth but so does all the other devices. I do not use this in the gym because the ropes were I climb are quite stiff and thick.
Conclusion, it is perfect for sport climbing but it does not replace an ATC

Pros: Works for both hands. Works for realy skinny rope, I have tried 9.1 and locks done realy nice. Feeds realy easy.

Cons: Absailing is hard. Heavy. Not good for top rope.

Would I buy it again? YES

I have a 10.5 rope and the sum feeds though pretty good!!

But dont use a 10.5 with the sum for leading, it gets stuck to much!

Great device! Most people would prefer grigri, but gosh, that sucks!
SUM can be used with both your hands equal, is smooth and won´t fail as grigri when you catch the lever by accident.
Heavy? ony if you´re too weak
Bad point? well, the manufacturer let´s it stand quite clear as to rope width. If you try and use an old 10.5 it will get tricky to handel the rope. otherwise, I will never use grigri again :D

at first it seems like a great belay device, unless ur using a 10mm or larger rope then it simply wont work. on the side of it its says its rated for a 9.1-10.5 which is just plain wrong. andything over a 10mm rope when load is applied the device will lock and not be able to belay...

I have a Gri Gri and a Cinch as well as just purchasing a SUM. I dont use ropes fatter than 10-10.2mm.

The SUM like any device takes a little to get the hang of (a quick 5 min tute will generally do it).
PRO's- Pays rope out fast, easy to take rope in quickly, bites hard and quickly onto the rope when the leader falls.

CONS-No good at ALL for rapping. Its easy to put rope in 'upside down'. Fatter ropes (ie >10mm) dont run that easily thru the device.

All in all its the best device for peeps who use sub 10mm rope. The SUM pays out faster than a Gri Gri and is easier to lower off the leader when they have fallen, compared to the Cinch.

I have sold my Cinch and only use my GriGri for rapping or new route setting.