RSpec-Rails is the most widely used testing framework in the Rails ecosystem, offering a behavior-driven development (BDD) style for writing expressive, human-readable tests.

It integrates seamlessly with Rails models, controllers, views, and request specs, making it easy to adopt across entire applications.

Setup is simple: add the gem, run the generator to create spec helpers, and start writing tests in the `spec/` directory.

Its strengths are readability, flexibility, and a mature ecosystem of matchers and plugins, while its weaknesses include a steeper learning curve for teams new to BDD. Alternatives include Minitest for a lean, built-in approach.