I do not sync up my address book between Outlook and my Tour. I sync only the appointments. I use BIS not BES. The individual address entries are still valid and in the address book. Just that the members of the group entry were dropped, and sometimes the whole group got dropped (but the group name itself is still in the address book).
Wyndtell > Sidekick > Sidekick II > 7230 > 8830 > 9630