PowerPoint -- This is mostly a case of lowered expectations... A lot of the older bugs I used to experience with PowerPoint have been fixed. It works better (perfectly) with some of the templates I use from work, especially those that involve integration with Excel. This should have been the case with Excel 2004, but it was not (the templates were made with Excel 2003 on Windows of course.) This fact alone gives PowerPoint 2008 a huge edge (for me) over Keynote or NeoOffice. I might still use Keynote if I was making a presentation for a conference or the like, but when doing "work", PowerPoint wins easily.
Entourage -- This is the really big winner. This beast works flawlessly with Exchange. I put in my email address and said "use Exchange". It prompted me for my domain and password, and everything just worked. All of the features of Outlook that I use work fine. Many of the features are much better on Entourage. Case in point, auto-complete on addresses. Our Exchange server stores names like Galpin, Michael. So if you start to type "Michael" then "Galpin, Michael" will be a hit on the "M" but not on "Mi" etc. Entourage 2008 is definitely smarter than Outlook 2003 (haven't used Outlook 2007 enough to say about it.)
I generally use Word and Excel more than PowerPoint and Entourage/Outlook, so I will definitely be giving them a workout soon. Finally, as for the UI... I like the switch to the metal look that is the de facto on OSX now. I actually like the ribbon in Office 2007, so I was a little disappointed that it was not used. However, there is very nice consistency between Office 2008 and Office 2004.
So kudos to Microsoft for making a great product for the Mac.