I've been on the largest one, which is in the Arts Tower of Sheffield University, UK.

http://en.wikipedia.org/wiki/Arts_Tower

There are a few safety features that help you to not get chopped in half. The front of each platform flaps up on a hinge, so if you got a limb under it while its moving down it just flaps up (the red board on the photo). Also there's a sort of trip wire along the upper edge (you can just see it in the photo) so if a limb of a rider is protruding while they're riding up and touches the wire the whole thing emergency stops.

We felt quite safe on it, and even perfected some fancy dismounting techniques (backward rolls and such) :)