Some friends and I get together roughly weekly using Jamulus. We're playing mostly jazz standards. We have gotten latency down into the 20 ms range, when we are all in the same town. That's the ping time. Add another ~ 40 ms that seems to be eaten up by my local computer and wired home network.
With that said, online jamming is hard work. I'm the bassist. Fortunately I can set my double bass aside and use my electric bass, so I'm not hearing my acoustic and delayed sound at once. The wind instruments and singer, not so lucky. For the bassist it's a constant chore to hold the tempo, leaving little room for anything expressive.
I'm glad to be doing this, beats not playing at all, but yet there is still no substitute for playing together in person.