I'm not a fan of RSpec; however, I also find Steak just a little too minimalist for me.
Occasionally, I want very badly to be able to render my Scenarios to plain text for communication with customers and other developers. I need the individual steps for that as well.
Where the example here maintains the steps (eg., Given, when, and then) as comments, I prefer them in the code.
So I wrote Coulda (http://coulda.tiggerpalace.com/) a bit over a year ago. Yes, it uses Test::Unit. While I am in the minority, I still prefer Test::Unit. It scales well.
When I say that, I mean in the sense that I can write almost any test using just "assert" and then add an error message for when the assert fails. I don't limit myself to using solely "assert"; however, when I'm not sure how to write an assertion, and neither shoulda nor Test::Unit gives it to me, it's easy to build it using "assert" and then wrapping it in a method (making it a helper).
And I still find Test::Unit code readable enough. Maybe that's just a matter of taste.
Dare I add: RSpec, although cleaner with the release of RSpec 2.0, is still a very complicated swiss army knife. Miniunit, included in Ruby 1.9.1 but also available as a gem for older Ruby installs, is the basis for Test::Unit as Ruby moves forward. That is, Test::Unit inherits from Miniunit in Ruby 1.9.
And Miniunit is far far simpler than RSpec.
Update: I'll consider rolling a screencast of how I TDD w/ Coulda as a basis for comparison.
(I recommend embiggening the video player.)