Google App Engine has a fairly restrictive Java implementation. It means that most libraries that you are familiar with would likely not work there. So you can't really just take a war file that works somewhere else and stick it on App Engine. They do provide a few nice built-in service like image manipulation.
The problem with services like Cloudbees and Amazon's Elastic Beanstalk is that you end up updating an entire war file that you build locally. This can take a long time.
Cloudfoundry only serves up deltas, but you still end up having to build the entire war file on your local machine ( at least for Grails ).
Heroku's approach seems much nicer, you get some initial setup pain, but then you can just use a standard git workflow.