I took a test drive on both products this evening and personally prefer Office 365 hosted Exchange solution over RackSpace's. Office 365 GUI is simple and clean, admin settings under one tab. Calendar is easily accessible. Draw back is, email archiving is a bit difficult to setup. I am sure it can be done, but couldn't figure it out. Best part is, no additional license for any of the Office products as they are already included.
Rackspace administration console is clunky, options are everywhere. Three different logins to manage users, mailbox and access archived emails. Office products are not included. OWA is your only online email portal, unless you setup your mailbox in Outlook. Rackspace includes mail archiving with a $3 per mailbox. AD integration is available.
Office 365 cost $8.99 / month
RackSpace cost $10 / month + $3 for email archiving.
