I use them all the time. Emboss right after you cover the cake and they should stay. The best stamps are the ones with more lines rather than the ones with lots of flat surfaces -- the stamps with more flat surfaces don't give good impressions.
I use the new rubber stamps on my cupcakes, without a problem. I roll out my fondant cut my circles and then emboss with stamp. Press a little hard to get your clean impression, just not too hard that it will go through the fondant. lol I don't know about it not being food safe and all but I could imagine that if it new there shouldn't be a problem. But I'm pretty sure someone with an FDA background will come on this topic and say everything you shouldn't do and why. So we'll just wait to hear from them. lol
Yep, there's always someone around to try and ruin our fun! No matter, I'll just direct them to the CIA book Cake Art, chapter nine! Or maybe Alan Dunn's book The Wedding Cake Decorator's bible, page 55. (Okay I'll stop now...)
Before I read it, I was happily stamping away...but now I'm only using the rubber stamps on fondant-covered cake boards, not on anything anyone will eat. I know it's not as much fun worrying about what's safe and what's not...but personally I would hate to cause harm to someone, even accidentally.