Anyone know how this compares to Heroku's built in postgres and rolling your own backups?

Heroku is on EC2, so its just as local/fast as their instances. Cost is probably a wash. What you're gaining is flexibility and control, you're also taking on some responsibility. Heroku -> RDS -> Your own EC2 instance is just a sliding scale trading flexibility for responsibility. I would start on the left and slide to the right as my application needs dictated.

In terms of price, the Ronin is 200/month, and you could maybe switch that for the smallest rds instance, which would cost about $80 at $.11/hr.