Even ADSL has high enough latency that raw X or
VNC are seriously painful. Modem links are utterly intolerable.
Raw X actually works remarkably well over a direct modem to modem ppp connection. There the hardware compression really helps the bandwidth, and latency is nearly instantaneous. (Ping time may be 100ms, but that's just the time it takes to actually transmit 64 bytes back and forth. The real latency is almost nonexistent.) Over a modem connection through an ISP and over the internet, both VNC and X are intollerable. Raw X being worse than VNC. X's main problem is, of course, latency and not bandwidth.
I have used NX over 56k modem connections (typically 45kbps) for full desktop, fullscreen sessions, and performance is remarkably good. I sure wouldn't want to use it all day. But it's pretty serviceable. Framebuffer strategies simply cannot beat the combination of aggressive, context-aware compression and aggressive caching. To get good performace, you have to be operating at the X protocol level.
NX memory usage is also significantly better. This makes a difference if you run a lot of desktops. My largest server (server in this context meaning the machine running the actual apps) runs about 65 simultaneous Gnome sessions.