Having worked with several solutions over the years, I found out that the unfortunate truth is that there is currently no solution on Linux that can match the features, speed, stability and ease of use of Outlook. And frankly, I'm pretty pissed because of this, especially since none of the available clients even supports open source Exchange alternatives properly (with MAPI support).
Yes, you have alternatives. You could use Thunderbird for email, Sunbird for calendaring and then some other solution for shared contacts etc, but that probably won't help you connecting to Exchange.
Outlook WebAccess won't help you much either, since Exchange 2007 will severely reduce the functionality if you're not using Internet Explorer.
If you have any influence on the back end (server side), I highly recommend evaluating alternatives like SOGO, Zimbra or Zarafa. The latter provides a WebAccess very similar to that of Exchange, but it also works perfectly on Firefox. I use it on a daily basis for all my work.
Of course if you don't have any influence on the server side, you're pretty much screwed. The only alternative I can think of in that case is running Outlook on a virtual machine or by using Wine. But I'm not sure if there are any Outlook versions that will work reliably on Wine...
If you want to know more on Exchange alternatives I can help you, but I must warn you that I have the most experience with just one of the alternatives ;-)