We used exmerge a few years ago to move from 5.5 to 2003. Had some major DNS issues to deal with at the time. But I really like 2003 now.
TIP, make sure your people don't have thier out-of-office turned on when you extract them out of 5.5 with exmerge. It caused a bit of corruption. If the users did, we found they couldn't turn it on any longer after the mailbox was brought back up in 2003. Took a while to find the tool to fix it later. But we had to manually go into their mailbox from some command line tool and remove their out of office flag. Good Times!!
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-sphilip
