Heroku rocks!
Everyone knows that deploying Rails applications is a pain. Love the development process, hate all the work of deploying your app (not such a problem if you have your own servers, sure). One of the great things about Rails is you can really, really quickly develop some handy little webservice or webapp just for personal, or small scale use. Clay Shirky calls these kind of apps Situated Software. But unless you have a nice deployment environment all set up, you might as well forget it - deployment will be more of an issue than development.
Heroku makes this problem go away. Quickly code up your service in the browser, click a few buttons and your app/webservice is live. This evening I threw together a handy webservice and some client side scripts in next to no time.
Another handy use of Heroku? Helping mentor other people in the use of Rails:
Heroku + ( IM || Skype || Phone ) = Really simple and highly iterative collaborative development
So, to recap: Heroku rocks!