In preaching through the book of John, I've used many commentaries and have found Carson's to be the best work out there. I suggest one consult Morris (NICNT and Reflections on the Gospel of John), Ridderbos, and occasionally Beasley Murray (WBC) for other helpful works. Kostenberger's BEC work echo's most of the above and should really only be used if you are looking for a summary (though his footnotes can occasionally be helpful).
Carson's commentary on the Gospel of John (Pillar New Testament) is excellent, and arguably one of the better commentary. Thoroughly evangelical, biblically solid and clear, understandable sentences. He processes the Greek fluently and translates effectively. One of the "staples" in the commentaries on John - well worth owning!
I am a layman, unschooled in Greek and Hebrew, who nevertheless enjoys reading commentaries on a fairly regular basis as part of my person growth in the Word. This commentary is the best I have ever read, with so much meat to savor it is a delight to read. I add my review to the others to let you know that although the author is obviously a scholar, he also knows how to write clearly, in plain language that anyone can understand. This book is special in my humble opinion.
Carson is always scholarly and evangelically trustworthy although at times a bit of a show off. He comes from the John Gill school of commentary. Spurgeon told his preacher boys if their goal was to simply fill time, Gill was their model. Gill would tell you several things a passage didn't mean, most of which you never thought of, and then told you what he felt it meant. If you are a preacher looking for good shank meat for your flock you'll find more bone, gristle, and sinew with Carson, but, you won't lack good scholarship. He does the leg work for you.