So if you ever have the pleasure of using this utility, feel blessed. I'm not sure of the cost on it, but it is WELL worth it.
I recently had the opportunity to assist with a small user migration (500 users) for a company that was being split out of a rather large organization. Since we arrived late (meaning we weren't brought in until the last second) this utility saved our asses. The beauty of the product is that it wraps all tasks that you normally have into a simple TaskPad type view and guides you step by step through it. It is pure genius!
With just a simple few configurations, we were able to transfer all users email, public folder data, and even accounts from old domain/exchange org to the new. It then created a contact in the old domain to allow for email to flow from the old to the new. It also created contacts in the new domain so that you could send to the old domain. You can limit this down to certain users quite easily so that you won't confuse the end user too terribly much.
There are some interesting things needed though. The account that you used to do the migration (I was creative and used an account labeled "Migrate") will need to have local admin access on both domains (this is the administrators group located in the default Builtin Container) and also the local admin group on the Exchange servers in both domains. It also needs Full Exchange Administrator access delegated to it for both Exchange Orgs.
However, granting that level of access actually frees you of an ENORMOUS amount of tasks out there.
I reccomend it!!!