i just moved from Entourage 2008 to Mail, Calendar & Reminders in OS 10.8. I was finally forced to do so because Entourage doesn't support iCloud/CalDAV and I needed some way to sync my devices and laptops.
Frankly, Apple is not up to par. For years I've been avoiding their suite because it's always been a mess, they keep changing it, and they keep removing features rather than building and improving upon them. It's embarrassing that Entourage 2008 (and its predecessors) outperform 2012 apps from Apple, and that it takes three Apple apps to not even measure up to what Entourage could do in a single, elegant, intergrated interface.
So, here's what you'll be giving up:
* coloured folder systems for organisation in Mail (welcome to Soviet-era monochrome)
* colour-coded categories in Mail (limited deployment)
* Rules that allow category sorting in Mail
* No more Project Center
--> Solution: MailTags, though not perfect, allows some of this functionality by colour-coding Projects
* Reminders do not show up in Calendar (no integrated To-Do list)
* Reminders lack due dates (this appears to be a bug: you can create due dates at iCloud.com)
* Reminders only appear in Notification Centre 24 hours before the event; no ability to set this
--> solution: QuickCal3 or Fantastical offer some workarounds though I have not tried them. For due dates, create a Reminder at iCloud.com, set due date, then use this as a template to copy-paste on 10.8. However still no ability to create due dates on iOS.
* Contacts loses Categories fields & colour-coding, websites, custom fields (Skype handles or whatever)
* Contacts is a stupid looking book thing
--> solution: I haven't looked into it yet, but you can tweak the look of Contacts with MountainTweaks
Ideally, I would've loved to have seen an Entourage update including CalDAV/iCloud support and a new, granular database for sync. Outlook is still no match for Entourage (many productivity features are missing, such as Redirect mail) and Outlook on Mac doesn't support iCloud either.
As for the Apple suite, it makes me wonder how Apple employees set their own tasks and due dates. I have a hunch they must use the above third-party apps because the current system would have you totally lost for serious productivity work.