If you're looking to build a commentary library, a worthy task for sure, it is difficult to do with the constant addition of volumes from various publishers.
I'd suggest checking a couple of sources prior to buying anything:
- bestcommentaries.com is a crowd sourced site that provides qualified reviews of commentaries and classifies them across a few categories.
- Pick up the "Commentary Survey" for both OT & NT from good scholars like Tremper Longman and DA Carson respectively. They offer valuable insights that will sort through the mess.
- Read, with a grain of salt, Amazon reviews. Also, check out the Society of Biblical Literature's book review site: bookreviews.org
That said, it is also important to understand what your scholarly acumen is and whether you're interested in deeper commentaries or more laymen oriented commentaries. There isn't a right answer here, just know where you are.
As for the question of the OP: If a commentary set is available and suits you, buy it. I've done that before. I've got the Word Biblical Commentary, New American Commentary, New International Commentary on the Greek New Testament, and the New International Commentary on the Old and New Testaments on my shelf. I've also bought particular sets like the whole JPS Torah Commentary on the Pentateuch as well as the Baker Exegetical Old Testament Commentary on Wisdom Literature.
Since I have all of these on hand, I am now buying particular volumes that I find helpful. Recently, for instance, I picked up the International Critical Commentary on Matthew because it is extremely good. I've also purchased a couple of other stand alone commentaries and volumes from sets to round out my work on Matthew.
My goal has been to have at least two of the best commentaries for each book of the Bible on hand (whether in print form or digital.) Using tools like the ones listed above sure do help with that.
Hope that answers your question.
