I find it interesting, that you're all seem to work in companies, where you can force your users to a limited mailbox size. In the company I'm working right now, we're setting no limits to our users, and so reached an approx mailbox size of 3GB per user on our ancient E2K3 machine and today some boxes are gaining 6GB in size on a E2K7 for the most active users. The approx size dropped to 2GB due to gentle clean up requests from the admin front..
Our public folders will break the 1 million mails barrier this year. Receiving a few hundred mails for different teams every day into the public folders and saving every sent mail ("send on behalf of" mail) in a "sent" folder for every team (thus still done by a linux frontend , I'm sorry ;) - filtering sent mails and redirecting a copy back into the public folders). Still got no time to try out the E2K7's internal filtering. Maybe it'll spare me some transaction logs by using them instead of the "out+back" into the system?
Cheers