I'm burned out (in a good way, that is) after two intense days of in-depth Exchange 2007 training. Delivered by Eileen Brown with assistance from James O'Neill and Exchange Ranger Greg. The sheer amount of information was overwhelming - 2 days of drinking from the fire hose.
It's clear that Exchange 2007 is a very different product with some aspects that seem odd at first, but which make a lot of sense once you think it through. E2k7 is 64-bit only (well for production), which means you can not perform a direct upgrade since you'd need an OS upgrade at the same time.
And there is no interoperation with Exchange 5.5. Now most 5.5 users should have upgrades years ago, but that's a different story. To perform the upgrade from a 5.5 to E2k7 environment requires going via E2k3 (Add E2k3 and AD, migrate the 5.5 mailboxes to E2k3, move the forest/Exchange into Native mod. Then add E2k7 server and move the mailboxes again). This sounds a pain, but it avoids the actual act of upgrading - which has always been an issue. Since the 'upgrade' is via data transfer that is well tested, this approach should be much more reliable.
The new architecture is awesome although I suspect old Exchange hands will take some getting used to the new architecture. By separating the various roles (hub transport, mail box, client access, unified messaging and edge) the installation is cleaner and I think will be easier to administer.
One of the cool things is that you administer Exchange 2007 by PowerShell (well you will when it all ships). At present, E2k7 Beta 2 uses an ancient version of PowerShell and if you install the latest version of PowerShell, you'll break Exchange. But this is just a small glitch in the schedules of two separate product teams and will be sorted out for RTM. This is a shame, since I can't play with integrating PowerShell, Power Gadgets and Exchange 2007. I'll have to save that fun for another day.
There are a lot of other new features, including continuous replication for higher availability, a range of compliance features, and an updated OWA (which works well with FireFox!). At the same time, there are a lot of features being removed. Some are just going away totally (e.g. OWA 2007 access to public folders), while others are being discontinued, but will still continue to work for some time.
If you are currently running some version of Exchange, you'll like the new version once the shock wears off! There is a large learning curve, but worth it I think. The new features are certainly ones you'll want.