my setup currently is:
evolution on desktop
syncevolution on desktop
account on mobical.net (a free syncml service)
syncevolution on N900
built-in syncml on my symbian phone

i sync one-way from the desktop to mobical.net and then transfer to both the n900 and my symbian phone. works very well.

i used to have the same setup with scheduleworld instead of mobical before, but i had technical problems with syncing contacts there, couldn't access their heavy and clumsy web interface and really didn't understand why i should pay for it when it caused such headache.