I've read several recent commentaries on John 8 and all agree that the story of the woman caught in adultery almost certainly wasn't there in the original manuscript. Is it original?
The latest argument in Evangelical theological circles is that we should not consider this part of the original gospel.
The ESV Study Bible is a Bible published in the last few years and all of the scholarship and notes comes from a wide swath of evangelical theologians and academics. Its study note on this passage provides a good summary of this view:
I have personally come to subscribe to this point of view.
Crossway Bibles. (2008). The ESV Study Bible (2039). Wheaton, IL: Crossway Bibles.
Under subjective analysis, you state:
"On the other hand, it doesn't add anything substantive to our understanding of Jesus, His mission, or His character."
I once had a discourse with a Jew who discounted the validity of the Passion because of his insistence that the Second Temple religious authorities were scrupulous in abiding to the Mosaic Law. I doubted that these officials were any different from any other officials, whether secular or religious, when crunch time comes, based on broad rendering of history.
However, this passage shows clear evidence that the Jewish religious authorities were not so scrupulous. Indeed, this incident is reminiscent of vigilante justice and lynching; which is not unknown by highly moralistic societies. And thus the passage speaks of rule of law and against such vigilantism.
Quick Subjective Analysis
Purely from reading the story, there's every reason to accept it as an authentic account of an incident in Jesus' life. Jesus' response to the woman and her accusers is among His cleverest, most merciful and profound moments recorded anywhere. For me, I will never let this story be forgotten.
On the other hand, it doesn't add anything substantive to our understanding of Jesus, His mission, or His character. We already knew that He was merciful toward sinners and able to out-maneuver the arguments of the Jewish religious leaders. The story merely illustrates Jesus' saying:
Was it part of John's autograph?
Clearly not. According to B. M. Metzger, “the evidence for the non-Johannine origin of the pericope of the adulteress is overwhelming”. In fact, most modern English translations note (as blundin's answer points out of the ESV) that the passage doesn't belong in John. Unlike many problems in textual criticism, it isn't the case that one or two manuscripts leave the passage out. Many manuscripts (especially the earliest and best) don't include it, many of those that do include it mark the passage with asterisks or obeli that indicate it was questioned by the scribe, and other manuscripts place the text in one of five other locations (after John 7:36, after John 21:25, after John 8:12, after Luke 21:38, and after Luke 24:53).
We can't know the complete history of the text, but the usual theory is that scribes wanted to include the story somewhere (and rightfully so) but there was no clear place to put it. So each scribe made an educated guess. Wikipedia points out that the earliest manuscript to include the text in either Latin or Greek is dated to the late 4th or early 5th century. The fourfold gospel (which includes John) was asserted as canonical as early as the 2nd century father Irenaeus, so if the pericope adulterae was not included at that time, it technically wouldn't be canonical.
Should we treat the text as "Inspired"?
When it comes to this passage I feel like we should follow Jesus' advice:
For that reason, I'm reluctant to cast it out of the umbrella of "inspired".
However, virtually every other word in the New Testament can throw stones at this passage. There are very few passages that can be questioned as inauthentic and none, not even the longer ending of Mark, has as little right to be included in Scripture as this passage. We can feel confident that the text of Matthew 7 is identical to the first autograph written or dictated by the original author. In comparison, John 7:53–8:11 is a fraud. Leaving that story in our Bibles does a massive injustice to every other word, 99%+ of which have been faithfully copied through the centuries.
What harm does this passage do?
I'll let Daniel B. Wallace take this question:
I heard one famous speaker say he didn't preach on this passage ever because it wasn't in one of the [supposed] oldest versions, and in a different place in another.
Now think about that logic.
It was in many, many manuscripts, but because one manuscript didn't have it and another had it misplaced, just because we think they were older, we're now supposed to throw out the rest? That means we didn't have a true Bible for a thousand+ years? I think the greater logic is that God preserved his word, and the majority of texts had these verses in them, so they are valid.
The church agreed that these verses were part of the canon, and it is only the textual critic-driven minority opinion that wants to throw them out.
A quote from wikipedia:
And another from Zane Hodges:
Regardless of whether or not this pericope was in the original autographa, it has been condidered canon for so long that to change it's status would not reflect it's historical position. Remember that canon is strictly consensus, and if you include those who've gone before us, it's still majority in.
One other factor to consider is that amongst non-practicing or non-Christians it is generally one of the very few verses of Scripture they know. They may not know John 3:16, they probably don't know "Go and sin no more," but try to mess with "Judge not lest you yourself be judged," and you'll have every secularist, agnostic, and atheist all over your case.
Even then to those not part of the conversation, this passage is thus canon, and there is really no way to remove it.