The Windows 8 Mail app is no good. It is constantly having problems synching to Exchange in our company. By far the most complained about Modern UI app. It doesn't handle multiple mailboxes well, especially the notifications. It appears to lose IMAP-related e-mail like our users GMAIL mailboxes. Windows 8 mail is making all of us in IT look bad.
On the flip side, the Outlook 2013 application looks like all the color was removed and the 3d effects are gone. The integrated search is now missing and the notifications are non-interactive.
Basically the Windows 8 mail app is too basic and doesn't appear to work. The new full Outlook client looks like a major step backwards as well. We're forced to stick with Windows 7 and Outlook 2010 for now.