I have been working pretty extensively with Cucumber for the last couple of weeks. In short, it is killer. You should be using it.
Having just RSpec/unit tests results in a lot of ugly trade offs between verifying the design and implementation of the parts (or units) vs the system as a whole. Using Cucumber completely absolves RSpec specs and unit tests of any responsibility for proving the system works. That allows you to use RSpec/unit tests as tools to improve the design, and reliability, of individual parts of the system without losing confidence in the overall systems ability to function acceptably.
If you are using Emacs i highly recommend cucumber.el. It has excellent support for editing Gherkin files and key bindings to execute scenarios, etc and view the output without ever having to leave the comfort of Emacs.