KaPi/KoPi for me.
It works wonderfully, has a lot of features - not that I use all.
I dont sync with anything at all - my Zaurus is a standalone machine in all, so dont know about the Synching, though I cann see a lot of sync options.
Only drawback - it doesnt interface with the Sharp Email program (not KaPi's fault - Sharp emails prog is closed), but you can use Opie mail in that case.
I used the Kompany's PIM too... its good and looks clean and more user friendly on the 5500/5600 devices than the KaPi/KoPi. but now that I have been using Ka, I havent felt the need to go back to the Kompany apps.
The great thing about Kopi KaPI is the taskbar applet which shows you addresses, Calendar events etc in one convenient starting point.