At the time my wife and I went to community college in our area, Joseph Campbell was all the rage among professors of the humanities. Campbell had a television series and supposedly ideologically influenced honchos of the Star Wars movies.
These days I hear much less of Campbell, though perhaps I am not in the right places to hear of him. He has since that earlier time passed away, and others have taken up where he left off. Some of these include persons he would probably not wish association with, such as the lay-level Christ-myth crowd. Nevertheless he laid certain groundwork that they would be unlikely to avoid.
Campbell's erudition is matters of history and pagan religions is not to be doubted. Works like his Mythic Image (which was a course textbook in my college days, and may still be) and Hero with a Thousand Faces (see more below) are chock full of data that the student of comparative religion will find useful. Not to say all of it. Campbell adhered still to the ideas of James Frazer (The Golden Bough), whose work has long been rejected (see comments by a classical scholar of our acquaintance here), and it is also clear by reading his texts that he tried very hard to force a mystical template on Judeo-Christian religious concepts, and did so by way of illicit generalizing. It is what we have seen used to more deleterious effect by the myth-crowd:
- Calling various figures "Saviors" with no attention paid to what they saved (from death? from a hard life? postage stamps?).
- Quoting the poet William Blake on the book of Job , who interprets it in mystical terms (i.e., Satan as "The Great Selfhood") which would never be found from interpreters familiar with ANE literature.
- Referring to any return to life from death in terms of a "resurrection" [29 -- done here with Osiris the Lego god!].
- Citing later Christian syncretism of pagan ideas and symbols without regard for whether such importation was in any sense alien to the first-century apostolic faith. (Though nowhere does Campbell lay out what he thinks is the significance of the parallels, practically speaking.)
- Citing Christian baptism  as a parallel to such stories as an Indian Goddess of nature emerging from the primal waters [! -- a rather misconstrued idea of the point of baptism; see here].
- In Hero, citing cases of women impregnated by gods as "virgin births" -- not one case being a matter of divine fiat creation.
In other places Campbell draws peculiar connections and leaves readers to reach an unspecified conclusion. A story of Qutezalcoatl being conceived  as an incarnation of the prayer-message of the Aztec Lady and Lord of heaven is said to remind us of Phil. 2:6-8, "Who, being in the form of God, thought it not robbery to be equal with God: But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men: And being found in fashion as a man, he humbled himself, and became obedient unto death, even the death of the cross."
Campbell's personal level of concept-association seems to have been quite broad! The conception of Wisdom that laid behind Phil. 2 is only remotely similar to what Campbell describes -- and by highlighting only commonalities, while ignoring vast differences (or in Campbell's case, he was perhaps unaware of them) does a grave disservice.
We also found it a little presumptive for Campbell to cite parallels between pieces of art removed from each other by chasms of time and space, such as a Buddhist carving of the first century BC to a German painting of the 16th century AD, or comparing 2nd and 7th century Indian and Chinese depictions of the Buddha being born from his mother's side with a fifteenth-century European work showing the crucifix, as a tree, growing from the body of a sleeping Mary! Establishing a thematic connection surely requires more than mere assertion and a few sentences.
So how, you may ask, did Campbell think he could get away with such wild paralleling? The answer is that Campbell did not follow the usual "copycat" crowd in thinking that Christians went out and literarily borrowed ideas after reading The Book of the Dead. For Campbell the similarities derived from Jungian archetypes -- if you will, mythical templates embedded in all human minds subconsciously. This is how it is that Campbell thinks 1st century BC and 16th century AD items can be regarded as parallels: It is not a matter of theft, but of common psychological source.
Campbell's dependence on Jung was a little more obvious in Hero with a Thousand Faces. But it was clear in his Preface that he was sensitive to the charge that he was overemphasizing similarities and ignoring differences. To this charge he petulantly replied, "The same objection might be brought, however, against any textbook or chart of anatomy, where the physiological variations of race are disregarded in the interest of a basic general understanding of human physique." Of course there are differences, he admits, but he was writing a book about the similarities, and hey, I'm doing this in the name of "unification, not in the name of some ecclesiastical or political empire, but in the sense of human mutual understanding." [vii]
That's designed to bring a tear to your eyes and a salute to the unity flag, but before we get too emotionally involved, keep in mind that Campbell's analogy is seriously presumptive. The differences are on more of a scale of comparing human anatomy with insect anatomy, or with Vulcan anatomy. The analogy begs the question of just how similar indeed the parallels are. Covering one's self by adding emotional components will not validate the practice of illicit generalization one bit. It is by the same token that our classical scholar made Churchill one of Raglan's mythical heroes (see link above) and our mythical Phonias J. Futz rendered Lincoln mythical.
Campbell's template of the "tyrant-monster" could just as well mythicize Hitler or Stalin, especially if we are allowed to explain vast differences as nevertheless variations on the same theme.
It apparently never occurred to Campbell that myths are alike not because of some Jungian mailbox in our heads, but because, as has been noted, there are really only about a dozen plotlines behind every sitcom or story ever written; because life itself is repetitive upon these very themes. Everyone is a "tyrant-monster" now and then. Everyone goes on journeys where they see unusual sights. Your next trip to Yellowstone is a psycho-workout of your Jungian template for fantasy quests, if you tell it properly.
And it gets more vague. Even the story of Red Riding Hood, swallowed by a wolf, is open to view as a parallel to a story of an Eskimo hero who called for a whale to open its mouth, and then darted inside when it did so and had a look around; or of the Zulu story of a woman and children swallowed by an elephant who found a strange land inside. Inquiry: How hard would it have been for there to have been no Jungian templates in our heads and for these people to come up with these stories completely independently? Campbell's theories are practically unfalsifiable and do not accept vast differences as a disproof.
Campbell seldom exhibited any opinions and most often let descriptions "speak for themselves" even if they did not speak well. (I.e., both Buddha sitting under the tree and Christ crucified are expressions of the "World Tree" motif -- I suppose the Romans had that in mind when they crucified people? -- and Mount Calvary is an expression of the "World Navel" motif -- never mind that there is no proof anyone in the apostolic or later church ever thought of it that way.)
But at one point it came through that he was set upon the idea that Judeo-Christian (and perhaps Muslim) concepts were a corruption of purer ideals associated with mysticism.  Campbell outlines the concept of chakras, or centers of energy, noting that of the seven that are in mystical thought, three are "modes of man's living in his naive state, outward turned..."
Most people, he says, have functioned only on the level of these three chakras and hints that the monotheistic faiths (he does not name them, but the implication is clear from his description) are designed for those living only with charkas 1-3 and not developing the rest. "...it is obvious that a religion operating only on these levels, having little or nothing to do with fostering the inward, mystical realizations, would hardly merit the name of religion at all. It would be little more than an adjunct to police authority..." If we wish to draw Campbell parallels, this sounds like Marx's "religion is the opiate of the people."
Campbell's ghost remains with us to this day, and it is well to recall that while his work was long on description and implication, it was very short on hard data and detailed analysis.