Culy has attempted to give us a handbook on the Johannine epistles that is not exactly a commentary, nor a detailed exegetical or syntactical study. Instead, his work is something of a top-level combination of all these things that has syntax as its focus, but also touches on interpretational issues without getting too detailed about it. As a result, someone wanting to walk through the grammar, parsing, and syntax of the Johannine letters will find this work to be predominately useful. A working knowledge of Greek is likely necessary to get a whole lot out of this work, but it is a work that is not so overly technical that beginners in Greek will be left out of the conversation.
Culy is often helpful on issues of syntax. While not agreeing with all of his conclusions, Culy often lays out the syntactical options in a particular passage in summary form in ways that are helpful and understandable. In this way, Culy does the reader a service in providing summaries of major commentaries regarding syntactical options. This should not be used as a substitute for the bigger commentaries, but it does provide a good springboard for someone who wants a general grasp of the issues at stake without wading through much more expansive and technical material.
Culy's major syntactical error regards the issue of deponency. Culy tends to side with the school that believes that true deponency is problematic, and that many middle forms in particular should be taken as true middles. Culy believes that in taking this position, the significance of the middle voice reemerges from the obscurity that was forced on it by the deponency concept. The problem is that in taking this view, Culy only interacts with the anti-deponency school and does not give us a hint of the arguments on the other side or interact with them at all. There are still many good reasons for maintaining the deponency concept in Koine that Culy glosses over completely. One would have hoped that on an issue that he himself regards as semantically significant, he would have bothered to present the reader with something better than a one-sided presentation that half-heartedly stacks the deck in a particular direction.
Lastly, one of the major areas where Culy engages in interpretational commentary regards the infamous passage in 1J 5:6-9. It is here that the reader will find (at least in this book) a rather extensive commentary on what the 'water and blood' mean. He concludes, a la Witherington, that the water refers to Jesus' birth, rather than his baptism. While syntactically this is a possibility (the aorist form for 'come' certainly seems to refer to concrete actions in the past), it is grammatically and theologically deficient. Culy follows Larsen (and Witherington) in citing John 3 as support for this idea. But in doing so, Culy is going against every major lexical dictionary and lexicon that unanimously state that the Greek word for water never refers to amniotic fluid. That Culy fails to mention this is very problematic and raises serious questions about the breadth of his scholarship. Further, Culy assumes that John 3 supports the Incarnation view, but this is not at all the case. Culy is appealing to a particular (and minority) interpretation of a highly disputed passage to argue for a particular (and minority) interpretation of another highly disputed passage to make his case. This is methodologically deficient, and he should know it. But secondly, such a reading is problematic in light of the letter's theological purpose - to combat a particular form of heretical Christology that proposed that the man Jesus was adopted by the spiritual Christ at his baptism, but later abandoned the man Jesus at the crucifixion because God can't suffer. Culy's interpretation renders the parenthetical comment of 'not by water only, but water and blood', incoherent. This comment assumes that the heretics had no problem with the idea that Jesus Christ came by water (at the baptism). Their problem is that Jesus Christ suffered on the Cross. If we accept Culy's interpretation that the Incarnation and the crucifixion are in view, this parenthetical comment of v6 becomes nonsensical, because the heretics would have had a problem with Christ coming in the Incarnation as well. Culy does not engage any of this data, so that not only is his conclusion problematic, the method he takes to get there is also.
So while the discerning reader will profit from this handbook, the reader does indeed need to be discerning. The reader should realize that the somewhat technical nature of the book does not hide the fact that a number of the conclusions reached are based on assumptions and starting points that are definitely not beyond challenge.