To be honest, I think both styles are good practice, but it totally depends on the context. For example, if I want to describe how the "#admin?" method is working, I'll use

    describe "#admin?" do ...

but if I'm describing a different method, that does different things depending on whether the user is an admin or not, I'll do the following:

    describe "#some_other_method" do
      describe "if the user is an admin" do
        ...

So you can mix and match both styles, depending on what exactly you're describing (and it's context).

If you're looking at the OP more closely, it even says this is only recommended for "describing methods".