Speaking of RDP though, I have yet to find a remote screen sharing tool that is as polished and that works as well on Unix as RDP does on Windows. I'd really like to be able to use something other than VNC to remote into my Mac from Windows.
I also looked at the SPICE protocol and its implementations [1], but again the user experience is poor over slow/flaky connections compared to RDP. The compact RDP data stream also makes it a good starting point for developing various RDP-based recorders.
In my opinion, RDP is a much-overlooked feature of Windows, and its superior implementation to any Unix alternatives I'm aware of is one of the reasons I still run some Windows instances in my environment.
The first method is the preferred option when connecting to legacy X11 applications, i.e. applications making mostly use of traditional X11 Render operations and X11 vector graphics. In this 'lightweight mode' NX reduces both the bandwidth usage and the HW requirements on the client and the server to the very minimum, by employing the expensive video encoding operations only when detecting that the server-side application is producing video data on the screen.
Unfortunately this 'lightweight mode' can't deliver games, a Mac or Windows desktop, as well as most contemporary Unix/Linux desktop environments. The reason, in the latter case, is the inefficient way these applications use the X11 protocol, as well as the inability of the X11 protocol to deal with modern graphics. In these cases, the use of video encoding is unavoidable. NX employs a number of different techniques to reduce the CPU consumption and the bandwidth needed, so much to often make video encoding advantageous even compared to the legacy X11 protocol compression. But, of course, the final results always depend on the screen content being deployed.
Disclaimer: I work for NoMachine
Windows RDP has been amazing for modern desktops. Prior to having over 8GB RAM and an SSD drive, I would often access Windows from by Mac via RDP. Since I am mostly Mac OS X, Linux or Windows visualized on my Mac these days, I seldom use it RDP, but I am always amazed at how responsive it is compared to VNC.
The best remote desktop I have used was Timbuktu for Mac back in the early 90's. At that time, Timbuktu was able to utilize the Mac's great support for 1-bit (black/white) and other low-bit graphics modes, and you could have a very good remote experience even at high display resolutions via a 14.4kbps modem. Putting Mac OS X into gray scale mode helps, a little but VNC performance still sucks.
I also remember seeing an amazing demo of a NeXT system that was able to push its Display Postscript based terminals to remote systems. Alas, Apple lost that when Display Postscript was replaced by PDF.