Our clients use either Google Apps or Office 365 for hosted email, but recently we found a serious drawback to Google Apps. A client using Google Apps registered a new domain name and wants to use it as their primary email and web domain. With Google Apps, you can receive mail as any domain you own by setting up a domain alias. However, to send mail from a new domain, you must create a new Google Apps account and migrate your data. To get around the issue without migrating, you can specify an external SMTP server (Google in this case), but this has to be configured for individual users and will break if the user changes passwords. The only real solution is to migrate to a new account.
With Google forcing us to migrate to change the primary domain, there is strong incentive to migrate to Microsoft Office 365 rather than stay with Google Apps.