We're using Heroku where I'm at and looking to move off of it for a number of reasons. Review apps are one of the great things about Heroku, but once your team is above a certain size its most definitely not free, or even cheap.

The biggest pain point I'd like to see solved (beyond just review apps) is the provisioning of good staging data. Ultimately we want data that looks like a subset of production data with PII scrubbed. Heroku doesn't give you an easy mechanism for getting this, and we had to sink a ton of time building this out for our specific use case.