I participated in a 2200 seat groupwise to exchange online migration, starting last August and finishing up in January. The largest impediment to the migration was the unreliability of the service and the complete lack of communication within the organization itself. I would often be on the phone with level 1 support in excess of an hour troubleshooting connectivity issues before they would check with their peers and realize a larger outage was in progress. The hosted blackberry service is complete crap as well.
Whomever came up with the acronym BPOS must have been giggling inside, because it truly is a BIG PIECE OF SHI# !!