When I was a young lad back in England my dad used to race cars. One track, Shelsley Walsh, was always popular with the kids because it had a playground there with swings and roundabouts etc. so the kids didn't get too bored. I was about 5 years old, 1978ish! and I remember seeing this thing when we arrived one weekend, it was about 5 to 6 feet tall with just one drop down into the grass. I rode that thing for the next 3 days but on the sunday it toppled over to the side when some kid got a bit too enthusiastic at the top and the ambulance crew came and took him away.
That was the last I ever saw of it and I've never seen another one since, If I had a back yard I'd buy this thing for sure. As for the danger factor, I don't think they are any more dangerous than a regular slide, just a bit quicker!