Given that Google Apps is being used by large enterprises such as General Electric, Proctor & Gamble, Google, City of Los Angeles, as well as by countless startups, I would say that Google Apps is ready for medium sized businesses.
Compared to Outlook/Exchange, the only risk is to to the jobs of people upgrading desktop computers with the latest additions & patches for MS-Office, and for those folks administering MS-Exchange and the computing infrastructure it resides on. (Of course that frees them up to do more useful IT work for the company).
The main "work item" for businesses is to get Google Apps integrated with your single sign-on system. But there are are many Google Apps partners to help with that.