(Tim, w/thanks to Jeff) Have I ever said anything about commentaries? Sure, but I'll have another hack at it.
When I left seminary, we had no money, so book purchases were mostly from used bookstores and resale shops. But I felt the need to have something "substantial" on at least one of the Gospels, so I took everyone's advice and spent about 40 of our limited dollars on I. Howard Marshall's commentary on Luke. "Stupendous example of evangelical scholarship at its very best" they all said, and I took the bait.
We moved to Pardeeville and I began preaching. Immediately, I looked for an occasion to use my most-excellent new tool and it wasn't long in coming. Choosing a text in Luke, I opened Marshall and...
Blech! Talk about a chip on the shoulder! It seemed the main thrust of his writing was wrangling with fellow New Testament scholars about problems invented by other New Testament scholars, few of which would ever occur to the lover of God reading Luke's account of the life and death of our Lord. Page after page of technical discussion that wasn't difficult to understand, really, but left the text entirely unimproved. There were few helpful insights and almost no timely applications or pastoral warnings. I found myself wondering whether this was really the state of the art in evangelical New Testament scholarship? Did all the scholars spend their lives arguing amongst themselves and was the thickest and most expensive compendium of those arguments the state of the art in commentaries on God's Holy Word?
Then I discovered Calvin's commentaries and would you believe my preaching improved? Top-drawer exegetical insights galore, and yet the thrust of every sentence, every paragraph and page, was the application of God's inspired words to the Church, the Household of Faith, the pillar and foundation of the truth. Since then I've learned a few things about commentaries.
First, if your work is in a place where some in your congregation keep up with scholarship and oppose preaching to the conscience and heart, I'm saddened to have to tell you that you have to own Marshall. In such cases, you'll want to have Marshall (or his equivalent) on hand and skim him each week to stay ahead of enemies of heart religion. Marshall, F. F. Bruce, Mounce--there are a ton of options and any of the myriad of seminary professor's lists of recommended commentaries will do for which one to purchase. Occasionally these scholars will give you an oasis of soul-water along with their Sahara desert of giggling excitement over fashion, but reading them will generally be a dry exercise.
Second, buy and use Spurgeon's little work, Commenting and Commentaries. It's superb and will serve as an excellent guide for commentaries published prior to the twentieth century. Spurgeon is not afraid to warn you off particular authors and works, giving very specific reasons that, at times, will leave you laughing out loud. It's hard to imagine anything this critical, and therefore helpful, getting out the door of an evangelical publisher today.
Third, sight unseen, buy the following:
I'll stop with those. For the vast majority of lovers of God and His Word, with the exception of a few classic commentaries on single books like Luther on Galatians, this is all you'll ever need to preach, lead devotionals, teach Sunday school classes, or lead small group Bible studies. Beyond this short list, Spurgeon's recommendations will serve you well.
One final note: Many of the best commentaries are copyright free since they were published prior to the twentieth century. Yes, there are some pious hustlers who claim they hold copyright to old Christian works because they've got a lot of sweat-of-the-brow equity in E-text versions they've scanned and OCRd, but trust me, they don't and you ought not to let them intimidate you by talk of lawyers and accusations of stealing. (For more concerning false claims of copyright by Christian businessmen, see this post from January 2006.)
Unless they've made a creative contribution to the E-text that goes far beyond such things as scanning, OCRing, footnoting, or hyperlinking the text's Scripture references, the law is clear. The E-text is copyright-free. Simply changing copyright-free material from hard copy to E-text brings no right to copyright that E-text and threaten with legal action those who use it and reproduce it without paying a royalty.
That said, many of the best commentaries are freely available for downloading to your computer. For instance, check this site out for E-text copies of J. C. Ryle's Expository Thoughts on the Gospels. This is just the beginning of what an enterprising soul will find with a little effort using Google.