iCal and the calendar feature of Microsoft Entourage can be used together–as long as you have iCal 1.5 or later and Mac OS X 10.3.5 or later installed.
If you have earlier versions of iCal or Mac OS X, when you try to add an iCal invitation to an Entourage calendar, an alert box with an error message could appear; an Entourage invitation received with the Mac OS X Mail application could appear as plain text instead of as an attachment; or, when you try to import or add an .ics file from Entourage into iCal by double-clicking the file’s icon, an alert box might appear.
If you weren’t around in the Entourage days, you really missed something. I ran it for years in college (as my school handed out Exchange accounts to students), and it was the hub for so much of my digital life.