I get asked this question from time to time so I thought it might be a good idea to post something on my blog.
The answer to this question is of course YES!. There are a number of features in Exchange 2007 that you just wont get if you don't have Outlook 2007 also.
With that in mind, take a look at the two following links
Moral of the story, if you want the best experience then you will need to deploy Exchange 2007 and Outlook 2007.
I run this combination at work and I wouldn't go back!