Heroku made the bold decision to design its routing technology to be optimized for things like Node and Go rather than Ruby. So if you are using Ruby on Rails (like RapGenius) Heroku's platform will simply fall short, both in terms of efficiency and flexibility.

XL Dynos let you throw money at the random routing problem which helps routing efficiency at the expense of spend efficiency (b/c they are a lot more expensive and scaling down to 1 XL dyno is still pretty expensive).

In my opinion, if you're using anything other than 1X dynos on Heroku you are compensating for some other part of your stack being broken.

We're probably months away from Heroku having some serious competition in the super easy PAAS space. Heroku's high prices have summoned competitors en masse, and by some stroke of luck Amazon has not really made a serious effort to encroach on Heroku's profits.