In preparation, I have been slogging my way through page after page of introductions to the Gospel of John. At the same time My wife has asked me to prepare some material on ancient Greek, Roman, and Jewish history and culture to aid her Bible reading. So, I've red\ad David Aune's masterful study The New Testament in Its Literary Environment (Library of Early Christianity). This is probably not a popular view, but I believe that these types of background study are indispensable - even more important to New Testament study than knowledge of Greek.
One of the commentaries I plan on using to study John is the masterful work of Craig Keener. I think he's one of the two or three best active Evangelical New Testament scholars. What I appreciate is his strong emphasis on background and his effort to place John within the proper genre for analysis. Keener, along with Aune, consider John to be a biography. Ancient biography could span from pure fiction to an account of a person's life that was firmly rooted in history.
Where does John fall on that spectrum? In John 21:24 the author insists on the historical reliability of his testimony. Clearly, this means that he is intending to portray Jesus in a way that was consistent with the life he actually lived. That is not to that he did not shape what he wrote or invent discourses, a procedure perfectly acceptable in ancient biography. In fact, inventing discourses would be a necessity, unless the disciples had truly remarkable memories. Speeches had to be made up, and were created to be appropriate to the setting and appropriate to the character of the individual. Ancient biographies would have been very boring otherwise.
So how historically reliable is John? There's really no way to know. To side with Dale Allison, we don't have enough independent sources to confirm or deny the historicity of many pericopes. The criteria typically used are of some value but I think are often misapplied. This is why Keener's and Aune's work is so important. Given the way biography typically was written in the ancient world and its connections to the synoptic gospels, there's little reason to believe that John invented large blocks of material wholesale in a way that would mislead us from understanding who Jesus was. He does indeed seem to have cast Jesus ministry in a very particular mold, but he seems to respect his source material. In my estimation, on the whole, John does portray Jesus accurately and give us real insight into his significance. Yes, John could be deceiving us, and there's no way we could ever know, but does the author of the gospel seem like a deceiver? Not to me.
 My biggest issue with Keener's work is that while depending on the work of Aune he obscures a distinction that Aune finds significant - that of biography from history. They were separate genres with separate conventions and expectations. When Keener discusses the historical reliability of John he sometimes draws analogies from ancient historiography.
 Keener discusses the ancient claims of the reliability of the memories of disciples of ancient teachers. While I believe that disciples could and probably did memorize teachers' sayings in a classroom setting or even public speeches, it is highly unlikely that they memorized disputes with other teachers or impromptu conversations. We need to deploy Keener's evidence cautiously.
 For example, multiple attestation in independent sources (e.g., of the crucifixion of Jesus by the Romans) does mean an even likely happened. However, given the paucity of independent sources, a lack of multiple attestation means very little as recently noted by deSilva.