I do, in fact, use modern X11 apps over the network literally every day. Some are better than others - if the programmer made the effort to actually gracefully degrade it can be a considerably better experience than the ones who just shoot a constant stream of bitmaps down the wire (which do work better on rdp, i remember once upon a time, I'd ssh to my linux box and set up port forwarding to a windows box on my lan so i can remote desktop to it, then run Xming from there... which is absurd that that actually worked better), but if you do it well, remote X is very nice to use.
The seamless integration of windows from multiple computers is a thing to behold. Remote Desktop is great and I like a lot about it, but even the "Seamless" rdp doesn't work as nice as X.