legal and no hidden fees in general.

the trick is: more relevant or irrelevant dental consultations could be offered to you once you get in with your free voucher, while your confirmation for those are required of course, though the extra consultations may be or may not be necessary at all, or may not be that necessary at least.  hold your mind, check your teeth based on how much the voucher is worth and ignore pretty nurse's pitches, that'll be all right. 

I don't know, but FREE stuff in China, I'm not really trustful. Especially dentists...

I got one maybe one year ago, good for a check up and teeth cleaning service.  Then the sales pitch began, like all my teeth must come out, repaired cavities needed to be redone, etc.  If you ignore those, yes, they are legit

Depends on how you got it and who gave it to you. I entered some contests recently and won some prizes from a different site but that site was trustworthy since they do have a magazine running in Beijing. If the person that gave it to you looked shady or was not employed at that place that you went to it's probably some type of come on. Don't trust people at those shows they had people selling fake tickets at the car show that I attended. I bet you the guy that gave it to you was standing outside wasn't he? Yeah, if he was it's not legit!