Similarly, you can "share" a file instead of attaching it. This let's both users alter the file without having to re-attach it to the email message without file-size limitations or the risk version-control issues - I really hate version control problems.
You can even share your desktop from the email window, letting users show each other things easily.
The company says it plans to integrate more Web 2.0-style applications into the Suite. We don't use Outlook here at bMighty so I can't give it a serious test, and IM functionality doesn't seem like that big a deal, but easy file and desktop sharing could make it much easier to work in a distributed environment.
The product is still in a pilot test phase, and the company site does not yet list pricing.