I'm currently trying to share the mouse/kb on my macbook (Leopard) with a Powermac (Tiger).
Using the 'latest' beta from sourceforge it's great-- but only with a normal mouse or the MB trackpad. Using my old Wacom Graphire3 causes some major acceleration issues and out of sync mouse ranges (ie, only a small portion of one screen will be available). I'm not concerned about the tablet functionality since I use the mouse 99% of the time (it's easy on the ol' wrists).
I tried the same setup with my Macbook and my work IBM T42 under Ubuntu and found similar results. And, IIRC, I remember VMWare Fusion doing the same thing.
Has anyone had any luck with a tablet and Synergy?