Chris, I own an X10, an X-S1, and the EF20 and EF42 flashes (I will round up an EFX-20 soon). Not too concerned with slave setups yet, but I am planning to get the Canon off-camera cable (you can also do the same thing with the Velo-brand Canon o-c cable, either available thru B+H or the other usual suspects).
This way, I can maintain full TTL control, which is what I want: The Fuji flash implementation is fabulous. I once did a test on a close static display, and the results from the X10's built-in, the EF20, and the EF42, were indistinguishable. As far as I know, there are currently no slave setups that carry thru this TTL operation. Anybody know of any? All I have found thus far is other-brand automatic flashes tripped by an on-camera flash from the camera. One of my favorite flash techniques is to hand-hold the flash off the camera (up high, to one side, etc.), so you can see my interest in the Canon cable.