What's different about Google Apps? Four important things. Here's the second:
2) It runs on the Google infrastructure: Google's hardware and system software.

The idea is that Google manages, for you, all of the operational adminstrivia, and they do it well: they have the resources to support multiple-redundancy, reliable backups, and scalability to any conceivable size ... unless your application is more complicated, or more popular, than the Google search engine itself, which just ain't gonna happen. Wouldn't it be nice not to worry about hardware failures, network failures, etc. Wouldn't it be nice not to need a DBA, a System Administrator, a Backup supervisor, a Disaster Recovery expert ...

And wouldn't it be nice to have predictable costs?

Of course, at the moment, the cost is very predictable: during the current 'beta' testing phase, it's free.