Heroku is the main reason why we moved from PHP to Ruby/RoR. My first encounter with RoR made me feel managing a Ruby server was a pain and resources never enough, but Heroku made me fall in love again. Now we have at least 10 apps with them.
Including mine. Love the service. It's sort of what I imagined hosting could be like but didn't think was possible. Anybody ever notice how cool the site design looks when you're working in the dark, late at night?