I find myself switching between Mail.app and Outlook for the Mac (Microsoft replaced Entourage with Outlook).
If you like Outlook for Windows, Outlook for the Mac feels like home.
The one thing preventing me from going Outlook full time is a bad ugly bug which is, if you get an iCal meeting invite from somebody and accept it, it never syncs it up to your Exchange server, so it doesn’t show up on your iphone, android, or even Outlook Web Access.
I NEVER miss appointments and this bug made me dangerously close to missing a conference call with a customer and therefore left a bad taste in my mouth.
But, now that I know about this bug (which many others have reported too), I make sure to accept those requests in Mail.app/iCal.
Once Microsoft fixes that bug, I would feel better giving Outlook my full endorsement.
Read more about Whats the best email client for Mac OS X: