Lesson 1: Don't run a pilot study!
Like any standard software product, the judgement of its suitability is unlikely to come from a pilot study which will just inform you about what users prefer (and most people prefer to stick with what they know). A better approach is to conduct a full change impact analysis and consider the needs of the business over individual preferences.
Lesson 2: Change the way you work.
The whole point of Google Apps is that you can work on email, calendar and docs from any location on any device. This is often a bigger challenge for the IT department and senior management than it is end-users! If you don't like the idea of your users being able to easily collaborate and use their own devices, then stick with Microsoft!
Lesson 3: You can do this in less than 4 weeks.
Once you've decided to use Google Apps, you can bring about the implementation (including the migration of calendar, contacts and recent emails from Exchange) in less than four weeks. Agree a realistic amount of emails to migrate. One month is usually enough if users can still access the pst archives from Outlook when required (you can always migrate archives after go-live). Remember to plan for shared / public calendars and start your communication plan as early as possible.
Lesson 4: You can continue to use Outlook (if you must).
A significant number of users will have an Outlook dependency, for example if their CRM product is MS Office integrated. This is easily worked around using Google Apps Sync for Microsoft Outlook. It's still important to train your users how to use gmail though, particularly if they would like to work on other devices from other locations.
Lesson 5: Stop panicking, and just do it.
If you've planned the migration and kept users informed, the transition will be much less of a nightmare than you ever thought it would be. If you are brave and decide to move the whole company over on the same day, you will probably make life easier for yourself. Users will adapt much quicker than you think and it's always better if you don't have to make your legacy and new system co-exist. Finally, and of course I would say this, but if you can afford to get a specialist with experience, it will always help!
Ray Allen is a Google Apps Certified Deployment Specialist at Third Way IT.