This beta will last until Feb 1 2007, which seems long enough for me.
Get the beta here:
http://www.microsoft.com/office/preview/beta/getthebeta.mspx
First thing that comes into mind : the design changed. And great it is. While in Word many options are hidden/stashed away (one could accidentally trigger a different viewing format with keyboard shortcuts and not know how to change it), they're organized here.

The insert tab makes it much easier and faster to maintain a document
http://users.skynet.be/alegis/office2.jpg
A bug here though was that if you customized your installation and removed clip arts, it will keep spamming the error in the insert tab.
Insert shapes, or mathematical symbols and equations with ease.
http://users.skynet.be/alegis/office3.jpg
Instead of digging into an options window with several tabs, it's all "within" the document.
Spelling check just got better with contextual spelling checks. Rather than just checking the spelling of each word seperately, it will attempt to find words that are used in a wrong context.

The new design makes it easier to work with, yet nothing THAT great if you're already quite advanced with XP. The graphical improvements come with a minor performance decrement when resizing the window, noticeable, but not grave.
Powerpoint though has a new feature called SmartArt. You can right click a bulleted list and get more advanced looking lists which again are very customizable and another way of displaying your data.

I really like this feature as the lists are pretty.
Excel looks just like the same as Word, and improvements are once again minor. There is a new listing feature where you can only show certain queries (like you would with an SQL database), Conditional Formatting feature. The other improvements (tables are what used to be lists) aren't really noticeable at first.
The other programs I couldn't be bothered with atm. All in all, it's mostly an interface change which should be a great relief for those inexperienced with the menus. Those advanced, well the powerpoint thing does add something, the other additions to Word and Excel don't and probably wouldn't make you switch from whatever you're using atm.
However, beta is free and until feb 2007. It is worth a try, only downside is that it's beta and not stable on some systems (may not install).
The install file is HUGE (all=1.5gb) , but customizable. Converters and filters, Insert equations, more silly popart, proofing tools, themes, Visual Basic for applications, fonts ...



