It's a rootles VNC, and has more compression options. Haven't gotten it to work with webp yet, but x264 works very well for me; I usually can't tell if it's local or remote.
This is different. This is forwarding on the application level, not the entire screen.
Isn't that what NX does as well?
Yes. If you're already using NX there's nothing to see here.
NX has a "rootless" mode, and a "desktop" mode. xpra has only the "rootless" mode.
That's just not true anymore. You can get a desktop mode by running Xephir inside xpra.
Bah now we're into semantics, but say you're running xpra on display :1. Running an Xnest or Xephyr inside that can't re-use the :1 display. Further, you can run multiple Xnests/Xephyrs inside an xpra session.
I stand by my statement that xpra has only a "rootless" mode, but I will agree that with Xephyr (which is a less-sucky Xnest), there is less of a difference anymore.
It may not matter to you, but xpra feels a lot more light-weight than NX. Installing/using xpra feels like tmux or ssh. NX feels like an enterprise software suite.
It definitely matters. I've always struggled with NX. VNC is easier, but slower.