To clarify, a ping of "---" means that the server is not ping-able but you can probably still join it. I just surfed through the unpingable servers and I was able to join most of them. Some routers can be automatically configured to forward ports and get a ping, some cannot and don't.
"???" ping means that you haven't pinged that server yet.
It seems to be working fairly well but there are a lot of different types of routers there so expect some compatibility problems.