When we were on Exchange 5.5, we TRIED to keep the user mailboxes to 50 MB, but there were a number of exceptions to that rule. Now that we’re on Exchange 2003, we’ve upped the limit to 150 MB per user. Obviously, there will always be exceptions to any mailbox quota you try to implement. Salespeople, for instance, seem to be genetically incapable of adhering to any mailbox limits and, since they’re out there making money for the company, we tend not to fight them. Same goes for the execs.
We’ve only recently migrated to Exchange 2003, so I’m still using the same backup approach (whole IS). What we’re doing, though, is breaking up the users into different file groups to ease the pain of restores. That allows us to take advantage of the Recovery Storage Group. I may still use brick-level backups for a small group of users, but that hasn’t been decided yet.