Generally I strongly recommend against using any kind of PST-based archiving feature if you care at all about the integrity of the old email data.
If users are having performance issues with Outlook opening (because their "Inbox" as 20,000 items sitting in it, etc) get the users some training about using folders to file messages.
If you're having space issues, either get users to go through their mail and delete things they don't need, institute a mailbox management policy (hopefully allowing users to store important items somewhere indefinitely, but making it a willful act to do so), or get a real Exchange archiving product.
I haven't used it (yet), but the Exchange Server Archiver from Redgate (who, incidentally, advertise on Server Fault) looks like a pretty well-designed and sane product. I've had a little bit of email conversation with them about the product and I got the distinct feeling that it was pretty thoughtfully designed.
A "solution" that involves PST files is just trading one problem for another. If you allow users to use PST files and store them on their local hard disk drives you run the risk of losing data when their local hard disk drive fails or their PC gets switched out. If you store PST files on a server computer then you battle inconsistent backups (when users leave Outlook running attached to the PST files) and you cause your differential backups to be gigantic (because Outlook updates the modified data on the PST file each time it opens).
It saddens me every time I see someone pulling email out of Exchange Server only to stick it in PST files on another server. It's not a "solution" and is the Wrong ThingTM