If I may add my 2 cents...
In my experience, both the above suggestions are correct. I've used the EXMERGE utility to bulk copy out a range of Exchange email accounts to PST files and then backed those files up to tape for archival (or, in my case, ultimately to import onto another Exchange server). It takes both time and bandwidth to do this, and it's not automated, but it works.
The Veritas for Exchange is somewhat expensive, but it rocks. It's stable, and it gives you a number of options for restoring. If a user blows away their Inbox by accident, the brick level restore will allow you to restore to folder level without server downtime.
I've also used ArcserveIT Advanced 6.61 with the Exchange option and it too provided stable/seamless backup with folder level restores. It was stable, once we got the right patches installed.
And test, test, test your backups. Restore to some old workstation box with a fat IDE hard drive - these days, almost anything will run Server and Exchange. When it comes to crunch time and the server is down, both the experience of restoring the box as well as the confidence that you actually have a working backup system are invaluable.
Hope this helps,
Doug