This is the second in the buying commentaries part of the book buying series...
Here I will give a handful of principles I live by when purchasing commentaries. Many of these principles are second nature by this point, so I hope to remember all of them!
1) Individual commentaries over sets: I buy commentaries for each book of the bible I am studying. I do not buy sets of commentaries. In other words, I will buy Nolland's Luke volumes from the Word Biblical series, but not the entire set. The reason behind this is simple. Sets are uneven in execution. Some in a series are exceptional, some not. Personally, there are only a couple sets that may break this rule. The Baker Exegetical, Pillar, and Greek Testament series are pretty solid. I personally own all of the Greek Testament series. Some other sets are close, but those series are the best as sets go.
2) Don't go overboard!: What I mean by this is that one can get a bit crazy and buy too many commentaries for each book. I spoke with a reputable scholar who received his doctorate from Cambridge and he limits himself at three for each book he studies. This may seem like a lot of commentaries for one book, as it did to me, but three can be topped pretty quickly. He added that once you buy three of the best commentaries for a given book, you will generally never have to buy another commentary for that book for the rest of your career, since commentaries do not change that much. I have broken this principle in the area of Luke/Acts simply because this is my research focus, and you will find that is true with most scholars in their areas of research. If you are in seminary, this is the best time to buy. In each of your classes, you will likely study specific books. Buy commentaries for each of these books, starting at one per book. This is what I did and it paced me well.
3) Be patient and find what you like!: We may want to pick up Carson and Longman and go buck-wild, but this is unwise. If you can use a library, make use of many commentaries at that library, and see which series and individual commentaries you jive with the best. Carson may suggest the Greek Testament series for a given book, but after much study and experience you find that you work best with Pillar, as some of my seminary buddies found; then don't go with Carson's suggestion, buy the Pillar book. These commentaries will be with you for a while, hopefully; find the ones that will serve you best as tools.
4) Be broad: Simply put - if you are going to buy three commentaries for each book, do not only buy commentaries that are ALL western-evangelical, and cutting edge modern. Get out and see the world! Buy one that is critical-liberal, African/Japanese/etc., classic. Broaden yourself!
5) When sets are OK: Only two sets come to mind that would be fine to buy. First, John Calvin's commentaries on the NT (the Eerdmans edition). You won't find that many current evangelical commentaries better Calvin on theological interpretation and overall comment on each book. He is simply classic and worth having. Second, Keil and Delitzsch on the OT are solid. Keil is sort of, meh, but Delitzsch is great. Many sets can be shelf-filler, which is not helpful to anyone, especially when you have to move! Let's not shelf-fill with our money. Support a missionary!
A few helpful principles that have guided me...
Others Book buying posts:
Book buying series, part 1: Bibliolatry - a Caution.
Book buying series, part 2: Bibliopegist Technology - a Blessing
Book buying series, Part 3a: Commentaries - WHERE DO I START?!
Book buying series, Part 3b: Commentaries - Principles I Live by...
Book Buying series, special edition: Dead Sea Scrolls