Heroku Helps Us Get Things Done

Heroku really sets the standard for how the usability of both the user-facing dashboard software is, as well as their CLI tools and APIs. They're predictable, well documented, and typically quite full-featured, and they've evolved considerably in the time we've been using them. That's married with great and extensive documentation, which is the final piece of the puzzle. That usability is what grabbed us initially, and it's kept us on the platform since then.

Customer support is typically quite high quality, with sharp front-line CS reps who are good at escalating any product issues directly to the developers working directly on the affected product. It's valuable to be able to speak directly to another programmer about an issue and not play broken telephone, and Heroku support usually manages to avoid that problem.

Heroku is fantastic at the beginning of a product lifecycle – in particular, because there are going to be some architecture decisions that will benefit from planning around a PaaS structure. Because of Heroku's fractional and low pricing, it's easy to start up on Heroku and scale up over time without incurring a huge up-front fixed cost.
At the other end, I'd imagine that larger organizations who have in-house staff doing DevOps might see a lot of duplication between those staff and what Heroku is doing to add value. At some point, the premium you're paying Heroku would probably prompt you to move those functions or keep them in-house.