As far as a introductory commentary goes, I enjoyed Dr. McGee's effort. His style is engaging, accessible, and interesting. One notable strength is what he affectionately and repeatedly refers to as "homely examples." Taking simple examples from everyday life, he goes on to illustrate the abstract theological point of the text. I think without exception I found these illustrations helpful in understanding his interpretation of scripture.
On the other hand, too often I felt certain passages were basically skipped over or glossed over. Additionally, certain theological hobby horses were emphasized over against the text itself. For example, extended discussions on the rapture are the dominating feature of the commentary for the end of chapter 3 and beginning of 4. Ironically, in the midst of this discussion he criticizes such methods, before jumping right back into it (although we teachers are probably guilty of this more often than we realize as well).
I think the benefit of this commentary is its simplicity and "homeliness" in presentation. This provides great access to the typical lay leader, and I think the preacher and teacher will find many helpful illustrations and insights as well. I think the weaknesses are lack of more technical discussions, which help show the strength or weakness of the theological truths presented. Such discussions are inevitably skipped in introductory commentaries in the interest of accessibility, but the other side of the sword is that certain theological emphases are presented as absolute, uncontested truths to unsuspecting readers. Such discussions also help stay focused on the main point of the passage, keeping us from wandering into other discussions that are perhaps important and biblical, but entirely foreign to the text in hand.