As some have said, this seems expensive. However, I would point out that the costs of using Heroku should be outweighed by the convenience and ability to focus on features. You are getting what you pay for here, the velocity you gain from not worrying about your infrastructure.

I do think that when you are first launching a startup, you should probably do everything as cheaply as possible, which probably means using a VPS or IaaS. As soon as your infrastructure becomes more complicated, you can use Heroku instead of hiring people to manage it for a while. Then see where your growth takes you -- you will probably find that eventually it makes more sense to hire a couple people to manage some IaaS or even renting racks and buying hardware.

As with all things in life, it really depends. I believe this particular configuration makes sense for companies who want to focus on product, not infrastructure -- at a cost of expensive hosting.