I like Behaviour-Driven Development very much. And that's my primary reason why I like RSpec.
I have another perspective as well about why I like Spec::Rails (RSpec for Rails). The specifications of Views are completely isolated from the Controllers and vice versa. This makes isolating and finding a problem much easier.
Also with the built-in Mock framework, Controller specifications and View specifications become more like Unit Tests (or Unit Specifications, I should say).
With stories to verify the interactions between two components acting as integration test/acceptance test, this approach is much closer to an effective software development dealing with the reality of software development such as frequent changes.