For a while I used the X10 RF sensors which spoke to my CM15a (with macros to convert the security signals) to transpond power line X10 signals out so that my INSTEON 2413U (with HouseLinc) could use the X10 RF signals in a roundabout way to control both X10 and INSTEON devices.
Yup, it was all getting confusing and slow so I just bit the bullet and bought a handful of INSTEON motion sensors, INSTEON door/window sensors and some INSTEON RemoteLincs (the old ones with loads of buttons). The only thing I miss is the old X10 keychain remote, but since I only used it about twice a month it's no real biggie.
I have found the INSTEON sensors to be more reliable and more flexible with the settings available and they are now driving all my X10 modules with no problems at all since I went "INSTEON RF" last summer.
With the X10 motions now running at $30 a pop, and the INSTEON at $35 it's not worth the fight unless have a whole bunch of X10 stuff that you're desperate to use.
If anyone wants a couple of CM15a along with a dozen ***eyes sensors, a handful of security motions and door/window sensors plus a bunch of assorted remotes you can't have them because they might end up back in use in case a deep nostalgia event that causes me to forget how much trouble I had making them work reliably in the first place