I have them, and they've always worked great for me. The cuffs are velcro, but I've never gotten out of them, nor has anyone else who's been in them. I did have one of the crappy little pieces of metal break when I had a couple of really strong dudes in them, but I actually was able to fix them better than they were in the first place.
I do recommend wrapping them around a headboard or footboard if you have the option. The only resistance that they offer is from them pulling on themselves, so with enough wiggling around you do eventually get some leeway. Wrapping them around something eliminates that. All in all, though, they're great. It takes seconds to put them on and you can leave them there indefinitely. I just shove the cuffs under the mattress when I'm not using them and pull them out again when I need them.