These migrations take a while, not because of the data transfer from one server to another. It’s also converting the database format and everything. 300 users took a while.
I also suggest (if you already haven’t) setting up your current exchange server as a bridgehead for 2007 to use, that way you aren’t bombarded with calls from people wondering why they can’t get into their e-mail when you moved their accounts over night. Eventually you’re going to have to change them over, but this will buy you some time.
I believe exchange 2007 comes with a transfer agent of some sort though. I wasn’t the one that did all of the transfers.
I’m working on an Exchange 2007 migration from Exchange 2003. There are 3000 mail boxes that need to be moved. Does anyone have suggestions and experience with this level of migration? I’ve used AD to move mail boxes in the past, but it is very limited.