The main problem with ssh -X is all your apps close when your connection drops. That is the main reason I always use screen or tmux -- I rarely actually use multiple screens, it just keeps my programs alive.
Here's my use case: I have multiple computers with an X server, but only one which actually has the graphical program(s) which I need to use.
So, on the "main computer", I set the DISPLAY to Xpra's :15 instead of the usual :0, launch the program in `screen`, and off I go.
Now wherever I need to use the program(s), I only need to re-attach the Xpra session, and it shows up at the local display. This allows me to keep the same windows open, but I'm taking them with me, wherever I go. Also, the programs keep running when I close the connection (or when it drops), so I don't have to a) start them again, or b) restore state.
More Xpra goodness: allows me to change the compression and algorithm on-the-fly; is more responsible than SSH X forward on slow/laggy connections; has a "connection problem" visual indication (the windows are overlaid with a spinner, instead of just freezing).
