I used the Y mouse for awhile but then I have switched to a much cheaper (and for me better) USB solution.

Instead of using Ymouse to merge the PS2 from the multiplexer and the switch I have used the monoprice's PS2->USB device (like $5) and a USB hub. High level (and somewhat simplified) overview:

I have an 8x PS2 switch, 8x PS2 multiplexer. The keyboard is plugged into the switch, the xkeys chain into multiplexer.
The output of the switch gets converted to USB and plugs into a 4way USB hub (one hub per computer). The output of the multiplexer also gets converted and connects to 2nd port on the HUB. I have a 4x USB vetra multiplexer that I use to multiplex a USB trackball. That connects to 3rd port on the HUB. Then I have an individual trackball for each computer (they are small) and that connects to the 4th USB hub input. The output of each USB hub goes to each computer.

This way I only have ONE USB cable per computer going from the "central spaghetti" that has all my mouse/keyboard on it. It is more manageable then the mess of PS2 cables before. This way also you can switch the keyboard between any character without loosing the xkey multiplexer input (at the same time).

Ziran