They provide a valuable service in terms of taking administration issues out of the picture and letting you just focus on developing your app.
But my feeling is this is not targeted at hackers who are surviving on rice and beans but rather those who are more likely to either have well-paying day jobs that can subsidize their startup hobby on the side or have enough VC capital to not bother with the mundane details of hosting administration and its associated time and financial burdens.
Not that there's anything wrong with that - Heroku has come up with an ingenious business model because it really does satisfy a need in the vanity hosting market where cost is not an issue - but belonging to the rice and beans category myself at the moment, I just find that I'm sadly not their target demographic - as much fun as it is to use Heroku's free option for little toy apps.
Deploying to Heroku really is a delight and I think we will see more services like them start up over the next few years.