I'm currently running Exchange 2003 SP2 Cluster on a Server 2003 AD Forest (in native 2003 mode), and we beginning to plan the upgrade to Server 2008 AD and Exchange 2010. We have two main sites, one middle-sized office, and a couple of smaller sites which have DCs (which may be RODCs after the upgrade). Currently all of our Exchange cluster is in my main site, but we are considering using the new datastore paradigm for load-balance/failover at the other large site, but this is not set in stone.

Right now we are in the information-gathering and planning phases. I am looking for input of any gotchas experienced while performing either upgrade, but especially the Exchange upgrade.

Gotchas? What surprised you? What wasn't documented? What said one thing but was misleading? (Confusing either in content or severity.) What is great or horrible about the new system? What worked well? What worked poorly? If you were to do it over again...?

(I know that this isn't so much a question that can be definitively answered, but I'm happy to reward insight and useful resources (not the Microsoft documentation, but Blogposts are welcome) with upvotes.)

UPDATE A couple items of note:

-We are not currently using OWA (currently only the admins), but it may become more of a consideration with iOS devices.

-We do have a small number of Blackberries in the environment (< 10%).

-In addition to the standard Exchange connectors, we have a third-party connector for Captaris RightFax integration.