Getting all of your accounts imported at once is fairly easy. You can import them all from a CSV file which I assume you can export from your Exchange host. You can read about the process here:
https://www.google.com/support/a/bin...57&topic=14586
There is also the migration process that the education edition can use. I have never done it, but will be a little later this summer. It is explained for you right here:
IMAP mail migration - Google Apps for Administrators
There is also this PDF file that goes in to much greater detail about the migration tools:
http://www.google.com/a/help/intl/en..._migration.pdf
Lastly, there is not currently contact sync (really one of the last steps to replacing Exchange IMHO) to get your shared contacts in to Outlook. You can do this through the web version of hosted Gmail by enabling contact sharing. Read about that here:
https://www.google.com/support/a/bin...18&topic=14590
There are methods to export global contact lists from Exchange. Again, never done it myself, but I do know that it can be done. You could use one of these tools to export your GCL from Exchange and then go through whatever the method is for reloading that in Outlook.
If you are paying for hosted Exchange are you using the free Outlook that is included with that? If so, you will be losing Outlook once you change hosts. The online version might be better for you any way you look at it.
I am working with the powers at the high school where my daughter just graduated from to get them off of hosted Exchange and hosted BlackBerry Enterprise Server and on to Google Apps and BIS. Yes, it is very different, but at a savings of just about $740 a month (not everyone with an Exchange account has a BES account) that is a lot of money for a private school.
They will be staying on Outlook as they were gifted with enough Office 2003 licenses when it was new that they can continue to use that. For them, that will at least give users familiarity.