Don Carson explains what a good commentary is and how to use it. You might be surprised.
Good all-round commentaries help readers think their way through the text—which requires adequate handling of words, sentences, flow of thought, genre, theological presuppositions, knowledge of historical setting, and, ideally, a commentary writer who is humble and of a contrite spirit and who trembles at God's Word. But most commentaries do not do all these things (and other things—e.g., interaction with some other commentaries) equally well. That is one of the reasons one is usually wise to consult at least two or three commentaries with different emphases.
Read the interview here and start thinking about what commentaries are lacking in your library. You can find Carson's book, New Testament Commentary Survey here.