OK, I just tried installing the ejamming.com software and signing up. It crashed. Twice. So I uninstalled it.
You also need 250MB/s uplink (and downlink) speed per person jamming. (NINJAM needs about half that per stereo channel, plus downlink of 160MB/s per person.) Given my uplink speed, I could only jam with three others on ejamming (whereas I've managed about eight on NINJAM - my CPU gives out before my bandwidth, I think).
I did some research on ping times. I'd be able to jam fairly okay with people in the UK and France. But further afield than that (e.g. Ireland or Italy) and it starts getting dodgy (over 50ms ping time, one way). Anything over 20ms can throw your timing (and that limits me to < 50 miles of London). I did a few "long range" tests, too: much of the US would be restricted to one or two others playing due to uplink speed restrictions; San Francisco was 201ms at 5350 miles away (if you say five words a second, 200ms is one word); Tuscon was 380ms at about the same distance; Yokohama was 292ms but the transfer speed was so low it would have been impossible; Auckland, New Zeeland was 0.6 seconds and 11400 miles away and barely had enough bandwidth.
ejamming is an interesting idea but it severely limits the people you're going to meet. And then they want to be paid on top of that! It's not even as if they're supplying any bandwidth - it's a peer to peer model.
NINJAM itself is working well, I'm just getting frustrated - I want to make music and so many of the guitar players just want to make noise (or "solo", I think they'd try to call it). And nearly everyone's a guitar player.