When I spoke on this subject (you can find the video here), our main focus was on the content of the gospels that are not in the Bible and their reliability as scripture. Though I believe it is clear that they cannot be relied upon in the same way as the Biblical gospels, the question remains, how should we use them?
I mentione that some are more like a novelization of scripture. The passion of Peter is a particularly good example. It takes the story of the passion from the Bible and adds all sorts of details that may or may not be true (and a little of outright contradiction). Just as it would be fun to read a print novelization of the Bible (there is a great recent one here), it can be interesting to look at these to help bring the story to life in our imagination.
The main thing that is helpful about these texts is to reveal to scholars the theological development of the church. By dating these ideas and looking at their prevalence, it can give insight into how different people in different parts of the world were discerning what was truly scripture.
The average believer who reads them should be careful, but can benefit greatly. By reading things you know are a bit off (or even way off), it forces you to clarify why. When you read about Jesus zapping some kid down who bumped into him in the Infancy Gospel of Thomas, you know it's not right, but why? Where in scripture do you find your support for that? If you are willing to wrestle with those kinds of advanced research and discernment issues, you can purchase a copy of all the existing translations of all the existing and hypothetical gospels here.
Or, just watch the video.