Over at The Complete Geek my friend Jered posted a really nice howto on remote X11 forwarding the other day.
Like many of the uses of Synergy, remote X can be extremely useful when you’re working with multiple machines, or even if you’re working with a virtual machine and need to run some of the applications on the host without constantly flipping windows back and forth. One other useful application of remote X can be if you’re using a machine low on resources, it can act as a terminal of sorts, running remote X applications from other workstations.
Jered also points out how useful it is if you’re standing with one foot in the Windows world and one foot in the Linux world, because remote X can make that easier as well.
Give it a read, it’s a great writeup. The post can be found here: Remote X11.