In reply to henwardian:
It's funny, I had the same reaction Rob had when first seeing the Ohm, dismissing it as just another "innovation" that was in fact pretty useless and a bit of a gimmick. Actually, it's VERY good and if you're a lightweight climber like myself, especially if you climb with heavier partners down the wall, you should definitely look into one of these.
When we did the test, myself belaying Rob at Awesome Walls, without the Ohm I was rather unsurprisingly yanked directly up into the 2nd quickdraw. Probably about 8 to 10 feet in the air and my plate was fixed against the draw. With the Ohm it was basically like belaying someone of equivalent weight. When Rob fell I was about a foot or so off the ground without the violent yank.
I actually recently had a pretty horribly experience at my local wall while belaying my brother. I had some slack out for him to clip, he was slightly above the previous draw although hadn't yet clipped it, he fell and I was yanked horizontally and vertically into the first draw. His feet touched the ground on rope stretch from about 35 feet. I'm pretty sure had I been using one of these that would not have happened.
I'm pretty accustomed to belaying heavier climbers, not many people weigh less than 55kg, but as soon as someone is significantly heavier than me it can become a real problem, especially at my local wall as the routes are short and there is very little margin for error. That extra tug upwards can make the difference between the deck and hanging on the rope.