I had used simultaneous-open TCP punching as a fallback for UDP punching when I was doing [0] and it did help in 10-20% of cases when (a fairly elaborate version of) UDP punching failed. That's on a scale of several hundred thousand mediated connections per day.
One caveat though is that it requires implementing a bot-like functionality in the clients, meaning that a mediating server should be able to tell a client - "create a socket, bind it to this ip:port, wait, wait... connect to that ip:port". Obviously, this is an ideal platform for DDoS attacks if someone ever manages to re-point clients to a rogue mediation server. So, yeah, it works well, but there are some not so obvious trade-offs.
I had used simultaneous-open TCP punching as a fallback for UDP punching when I was doing [0] and it did help in 10-20% of cases when (a fairly elaborate version of) UDP punching failed. That's on a scale of several hundred thousand mediated connections per day.
One caveat though is that it requires implementing a bot-like functionality in the clients, meaning that a mediating server should be able to tell a client - "create a socket, bind it to this ip:port, wait, wait... connect to that ip:port". Obviously, this is an ideal platform for DDoS attacks if someone ever manages to re-point clients to a rogue mediation server. So, yeah, it works well, but there are some not so obvious trade-offs.
[0] http://swapped.cc/hamachi