I originally struggled to keep Outlook when I moved to the Mac, I had been using it since the beginning. Eventually out of frustration I gave up and went to just using iMail.

After a while, I've gotten used to using iMail and it's snappy and reliable. I miss (terribly) the Calendar integration in Outlook and the easy of finding common meeting time with people via the free/busy and being able to just drag-and-drop an item right into the calendar was great. I'm not a huge fan of iCal. These are things you can still do in iMail/iCal, but the are much harder and not nearly as nicely integrated.

The Mac version of Outlook was never great, even the latest version seems a little funky and the synchronization never seems to work as I expect.