They are a lot of fun. I don't think I would trade my skateboard for one at this point, but they have potential.
I do worry about the kids though. The falls are nasty. People tend to stand completely upright with their legs straight often falling off the back. This makes it very difficult to avoid your head making contact with the road.
1. The board tends to runaway from you unexpectedly
2. You're not in a crouched/skating position which makes rolling off the impact easier
I suppose they are no worst than skates/blades in this regard. But skates are much easier to control and less likely to unbalance you when you make a subtle mistake.
Here is a compilation of crashes:
Be careful with the cheap Chinese knock-off models. As a benchmark, boards costing less than about $300. There have been quite a few go up in flames:
