I have a nokia 6500 slide and I had the same problem regarding the sync of the Contacts with iSync. The agenda is sychronising well with iSync so I keep doing the synchronisation of the agenda only with iSync. To manage the contacts and more I found a useful piece of software (around 34 Euros) :
http://www.novamedia.de/epages/e_produkte_macfl.html
There are two "styles" of synchronisation with this software, I don't use the synch manager which is unstable. I keep my contacts updated on the phone itself, then copy this to my computer using fonelink, so I am able to read them on my sreen instead of each time taking my phone. After it is always possible to copy the vcards from Fonelink to the Apple address book.
This is not a prefect solution, but it works...
I hope this will help other users.