At Microsoft, we "dogfood" our own technology. What that means is that we start using pre-release software. The result is a win/win: Microsoft employees get value from the latest and greatest technology; the product team gets to work with another customer to validate the software in its beta stages.

I use Office "12" Beta 1 on my production laptop. Sure there are a few kinks since it's beta software... but the value-add is definitely worth it. Here are a few of my personal favorite features in Office "12" Beta client that I can't live without now:

Outlook12
   search - the search in Outlook12 is simply stellar. As you type, you get search results... and there is query hit highlighting! Needless to say, it's very fast and efficient. My personal favorite is support for keywords... so I can say things like from:arpan subject:hello. 
   tasks integration - all my tasks in one place. I can view my tasks w/ my calendar and drag and drop tasks around! And if I "forget" to do something, it will just move it to the next day!
   rss client - I can subscribe to RSS feeds! :-)
   integration with WSS "v3" - we have some WSS "v3" sites... I can take doc libraries and tasks offline.. .and open SharePoint calendars and treat them just like my personal calendar.
PPT12
  graphics engine - I held a meeting with some of my colleagues... and I created some great looking slides in a matter of minutes. very cool.
  integration with Office "12" SharePoint Servers - you can insert a slide from a Slide library... :-)
  live preview
Word12/Excel12
   the best improvement here personally is the UI - I can find commands a lot more easily... in fact, there are commands I never new existed in Office 2003
   Excel12 conditional formatting - wow
   Formatting, live preview
   Slider - I can zoom in and out of the document/spreadsheet very easily

There are a bunch of great WSS "v3" and Office "12" SharePoint Servers features that are my personal favorite that I've already blogged about... but the one feature that recently got exposed internally was the next version of SharePoint Search. Sure, the relevancy is significantly better for enterprise search, there are great User Experience enhancements (query hit highlighting, query correction, duplicate collapsing, et cetera)... but the one thing that absolutely takes it to another level is how the next generation SPS search technology allows you to search business data. Data trapped in LOB systems will be searchable in Office "12". We've deployed it internally to search our CRM database so I can find information on a customer very easily. :-) I've used this several times over the last couple weeks to find a specific account manager for a specific account. It's jaw dropping amazing.