In Japan right now and have been using one all week, sometimes with a headset, sometimes plugging in the RJ11 jack from the hotel phone into it just so you have a "normal" phone to use.
So, I've had one (now two) since the beginning. I find that the outbound calls work almost all the time, about the 95% I mentioned earlier in the thread.
Inbound, however, is total hit-or-miss for me. My local phone number one has only worked a few times since I've had it, right now it isn't working, for example. I can make outbound calls fine, but inbound just go to the voice mail.
I contacted support before and they said that it is probably a firewall issue, which I suppose could be true, I haven't tested it to know for sure and since I use it pretty much for just outgoing calls, no big deal.
Another thing about the MJ is that when calling the US you have a nice US looking caller-id number that people can recognize, unlike Skype.