Yes, you can use Sync'Em in conjunction with Entourage but you must ensure that Entourage's syncing is turned off (unless you are using Entourage without Exchange — i.e., if Entourage is being used as a stan-alone main client for non-Exchange email servers).

Entourage's syncing can be turned on/off via its preferences.

Specifically, it's always dangerous to sync two of the same resources "together" via multiple sync'ers. In this case, the two resources would be Exchange contacts with Address Book contacts and/or Exchange calendars with iCal calendars and the two sync'ers would be Entourage and Sync'Em. This danger is true whether it's Entourage and Sync'Em or any other sync'er programs syncing the same two resources. For a more complete explanation, see our Knowledgebase article on this topic.

For safety, if Sync'Em determines that Entourage appears to be installed, it will show an advisory message once after each new version that's installed.