I just responded to a post on serverfault.com concerning hosted email solutions vs. in-house solutions. I feel strongly that the less I have to technically manage, the better job I'll do of providing services to my users. It's important to distinguish between technical management and executive management. Executive management is the process of providing oversight of technical services such as email from a business standpoint and technical management is about the process of configuration, patch management, and so on. The technical management of any server well is a lot of work, but it's especially challenging to manage a mail server well. You have to consider availability, security (including spam filtering), keeping your domain off of email blacklists, mailbox management, backups, and patch management. (Did I forget anything?) You can get shared calendars, messaging, and similar services through Google Apps for Your Domain (GAFYD) or other providers. At $50 per user per year for 40 users, for example, it's much less expensive to use a hosted solution than to invest in the required equipment and devote your time to managing in-house email services. Using a hosted service frees you from the minutiae of technical management responsibilities and allows you to focus on managing email from an executive oversight perspective.
You must also consider security and control. For example, if you work in health care or finance, security and privacy and/or regulatory concerns may limit your ability to use a hosted solution. You may also want to maintain a higher level of control over a particular service than what is allowed by a hosted solution provider. Another factor to consider in choosing a hosted provider, especially GAFYD, is the issue of data mining. Data mining is a fact-of-life for anyone using the public Internet. Providing usage data in aggregate is part of the price we pay for services such as GAFYD. If you don't like the idea of being "data-mined", don't use services like GAFYD.
Of course, as your organization grows, the cost of the hosted solution will also grow. At some point, it will probably become more cost effective for you to bring email services in-house.