I had given some thoughts to this before. Using DigitalOcean (with perhaps Tutum) is cheaper if you are running a handful of small apps.
However the convenience of using something like Heroku can be underappreciated. I once lost data (docker data volume) on my Digital Ocean instance because I was too lazy to set up backups. With Heroku (and their Postgres service) I wouldn't have to worry about these "administration" stuff, and focus only the idea and the app ... which is exactly what I enjoy doing.
That said, for testing and stuff for personal use only the 18h deal is fine (although technically trolls could just go around calling random heroku instances every half hour, burning through the awake-time on free instances). And I can understand that they want to limit free usage.
And of course Redhat OpenShift still gives you three apps for free ;)