Yes and no, which means yes (in that order).

The "database" for Entourage, Outlook and OWA is Exchange — actually, these are all "clients" that utilize the services of Exchange servers.

So, effectively, Sync'Em syncs with Entourage/Outlook/OWA — i.e., Entourage/Outlook/OWA all "see" the contact and calendar information sync'd with Exchange by Sync'Em and Sync'Em "sees" and syncs the contact and calendar information added/changed/deleted via Entourage/Outlook/OWA (because they're all just telling Exchange what to do with items that reside in Exchange).