Nothing is wrong with Test::Unit or Rspec or any other testing framework. I happen to like Minitest and want to use it in my Rails app. Also, the testing infrastructure in Rails 4 is built on Minitest and so minitest-rails enables its use in Rails 3.

I dislike where Rails places the tests by default, so I wanted something to support a more discoverable structure.

Its a testament to how Rails is architected that its to easy to plug in an entirely new testing framework. Yay Rails.

I like it because it is simple, fast, and straightforward. It also has nice additional features like benchmarks and mocks.

It's a step up from test unit and less "magical" than rspec.