You can display your to-do items, along with your calendar events, in the new My Day widget. Visible all the time, whether you're in Entourage or not, My Day lets you see what's on your plate at all times. It's also live, so if you click the check box next to an item, it marks it as complete.
Entourage 2008 adds some very useful features for those connected to a Microsoft Exchange server. For example, anyone can schedule an event and invite people to it by choosing their names from the Contacts list. But those in an Exchange environment can see other Entourage users' schedules to make sure that everyone' is free. Entourage with Exchange also lets you set up an out-of-office message to be automatically sent during the time you specify.
Entourage is a solid personal assistant (it doesn't seem enough to just call it an e-mail program), but there's no compelling reason to use it instead of the combination of Mail, iCal and Address Book that come free with your Mac, unless you're in an Exchange environment. Once you get Office, of course, you might decide you prefer handling e-mail, contact management and scheduling in one program rather than three, or you might really like the My Day widget, or you might prefer Entourage to the Apple alternatives. But don't buy Office just to get Entourage.
From some angles, the overall question of whether you should upgrade to Office 2008 is a no-brainer. Office 2008 is as close as you can get to a universal app, and all of the changes I explored are improvements or at least potentially worthwhile additions -- why not get it?
Well, maybe you've never needed compatibility with Office and have found alternative programs you like better, or you're perfectly happy with one of the open-source alternatives (or you just hate Microsoft). Certainly, if you've been able to get by all this time without Office, there's little reason to buy the 2008 version.
But if you do use Office, plan on getting Office 2008. I'd recommend waiting until you get an Intel Mac if you haven't yet, because of the performance hit you'll take compared with Office 2004 on a PowerPC Mac. But aside from that caveat, you'll find that Office 2008 helps you get your work done more quickly and easily than before. You're also likely to start using features that were always there but were too much trouble to bother with, such as using alternating colored rows in your tables or graphical presentations of ideas on your slides.
There's something to be said for a new Chevy, after all.