You can in fact mix and match - you can use Google Apps on top of a corporate system such as Exchange for only specific users. It's unlikely to save you any mon

You can in fact mix and match - you can use Google Apps on top of a corporate system such as Exchange for only specific users. It's unlikely to save you any money since you'll then need someone to run it, but it's possible. As things stand at the moment the company I'm working for is in exactly that boat of some staff depending on email, and others only needing to retrieve a single email on each day they're working. We're working around it right now by just getting them to give us their personal email address. That probably won't scale long term, but for now it does the job, and if we get to the point of needing more then I may just throw together a quick and dirty mail server for the people who don't need the convenience of Google Apps syncing between devices for them.