Besides the obvious cost savings, I like the idea of being able to access the apps and the documents from anywhere via a browser and being out of the business of having to perform and maintain backups, and being able to support users that might be using platforms other than windows. But really, the cost argument is really, really compelling. You might want to look at some of these resources below.
No, I don’t work for Google… but I was doing some research on this very thing today.
A short video that shows some of the WOW features of Google Apps.
Google “Businesses Share their stories” reference deck – includes industry specific video of customers detailing their decision processes, implementation strategy and cost savings.
The Google Apps Channel – high quality video with customers discussing their usage of Google Apps
The Google Enterprise Blog on blogspot.com – directly from the development and product management teams at Google. See what they’re working on and what new customers have recently selected Google Apps as their platform to deliver end user applications
DitoWeb – website of a popular Google Apps reseller / integrator – includes client testimonials, technical information, and newsletter.
Hope this helps you out.