Do you use attach or call variables by name or slicing

Question

Many beginner R books and guides begin with the practise of adding a data.frame so that variables can be called by name. I've always preferred using $ notation or square bracket slicing [,2] to name variables. That manner, I may utilise numerous data.frames without causing confusion, and/or use iteration to call columns of interest sequentially. I noticed that Google just published R coding guidelines, which includes the following sentence.

1) attach: don't use it

What are people's thoughts on this practise?