Have you looked at Padrino? I recently experimented with it and was surprised that it was an extremely awesome middle ground, to the point where I'll probably use it for my next project. It's very fast, close to Rack and easy to use with Rack middleware, has Rails-style view helpers, has sinatra-style controllers, very simple and elegant mountable apps, a focus on DB agnosticism, among many other things

Question: Have you experience with it on heroku? Does it work well there? Any difference between its behavior and the "normal case" of RoR as far as what heroku is designed for to your knowledge?

(Like I've said, got nothing about ruby, just don't feel rails is a good fit for the stuff I do; something lighter weight like this could be a good substitue on heroku).

I haven't, but I know it works fine. Heroku is more of a Rack host than a Rails host, so any Rack-based framework (AFAIK, all Ruby frameworks at this point) run seamlessly on it.