I have periodically used Heroku a lot, but I do have one gripe about them:
I tend to run a lot of low traffic web apps, apps that run well on Heroku's free tier (and not minding the occasionally loading request delay). However, I just don't feel comfortable using a service a lot that is free: being a freeloader. (Compare to Google: they make money off of me because I click on interesting looking ads.)
If Heroku had a cheap $10/month 1 dyno tier (that might have a perk, like staying resident, not swapped out) that was restricted to a lower number of requests per day, then they would get a lot of my business running my experimental projects.
I have considered paying for a second dyno that I don't really need, but $30/month each for a lot of experimental web apps does add up. Not too off topic: I just wrote about how I create a personal Heroku-like experience on a VPS: http://blog.markwatson.com/2014/09/setting-up-heroku-like-gi...
In any case, I think Heroku provides a great developer experience.