Yes - We have done it for many organizations including Pipeline Micro, ClearFuels, PriceHorne, etc. The experience has been overwhelmingly positive. The migrations go quickly and people are amazed by the improved user experience.
The migration is relatively straight forward. The main challenge is in training people who are used to using deep folder structures to organize their email with Outlook to adopt the new paradigm of labels and lightning fast searches in Google Apps.
We find that many users, especially the younger crowd, tend to require almost no training because they have gmail for their personal accounts. Sometimes it takes folks a while to leave Outlook behind, but they can always continue to use legacy clients if they choose.
We're also starting to integrate third party apps that sit on top of Google Apps such as Manymoon for project management.
I've had great success with GAPE since I first adopted it over two years ago. For me to let my 8 years of .spamassassin training go was quite a leap of faith, and their IMAP service has never let me down. You can keep it rather transparent on the client end if you choose, or you can expand their comfort of interacting purely with a web browser.
It was in 2008 that I began talking to the Google Apps team and UH's College of Education about online curricula to help the adoption in the DOE. I brought their Apps for Education team into the DOE in May of 2009, and I continue working with the most innovative schools in our DOE who are migrating from more complicated infrastructures than the standard Exchange stack. If you need some assistance, I am at your service.
Hi Brian - I agree that email and calendar are the killer apps. I would also add chat because its nicely integrated with everything else. Its the standard at our company. Google Docs and Sites aren't quite at the same level but they are getting better all the time and for certain tasks, such as real time collaboration, they are very good. Many new features have been integrated over the past few months including the ability to add binary files to folders.