I just completed Enoch 1:1-36. That's the most often quoted portion concerning the Watchers and Nephilim. I'm still collecting my thoughts and will post about this soon.
As I said previously, the Book of Enoch is strange. It fell out of general use before 300-400 AD when the RCC consolidated the canon as we know it today but was preserved by the church in Ethiopia, and since Ethiopia was geographically and culturally isolated, at least regards Christianity, it was not pressured to conform as were the Roman territories. That's why we have this book today. Unfortunately we don't have the texts in their original language - aside from some fragments in the dead sea scrolls.
The text describes Enoch as a "righteous man, whose eyes were opened by God". (En 1.2) It then goes on to describe how he was shown a vision from God, but it doesn't get straight into that vision but first takes a detour around the Watchers and Nephilim.
The Watchers are described as angels who were tasked with the job of watching over mankind, but instead of performing their duties as commanded they lusted after human women and took them as wives. The watchers began to "defile themselves" (En 7.1) with human women who "bare great giants, whose height was three thousand ells" (En. 7.2). For reference, an "ell" is about the size of a "cubit", which is approximately 18 inches tall, meaning these giants were 4,500 feet, or 1,385 meters tall! The giants were said to devastate the earth, first by consuming all that man produced, then by consuming man, and animals, and even each other. Yes, the giants were cannibals.
In addition to birthing giants, the Watchers taught mankind knowledge of metalworking and weapon making, of makeup and gathering of roots, of enchantments, astrology, movement of clouds, and the earth, the sun and moon (En. 8.1-2).
It was at this point that God sent down angels from heaven (Michael, Uriel, Raphael and Gabriel) to set things straight. They captured the Watchers and bound them in a pit of darkness, covered with rocks to await the final judgement (En 10.4-7, 11-15). The Giants were made to fight each other and kill one another (En 10.9-10) and also be devoured by the flood (En. 10.2-3).
Here's where it gets really interesting. In Enoch 15, God is telling Enoch what he is to tell the Watchers regarding their fate. God says that "the giants, who are produced from the spirits and flesh, shall be called evil spirits upon the earth, and on the earth shall be their dwelling." (En. 15.8). He goes on to say "And the spirits of the giants afflict, oppress, destroy, attack, do battle, and work destruction on the earth, and cause trouble: they take no food, nevertheless hunger and thirst, and cause offences. And these spirits shall rise up against the children of men and against the women, because they have proceeded from them." (En. 15.10-11).
That last bit seems to be describing what we now call demons. The fallen angels are trapped deep in the earth but the disembodies spirits of the giants are permitted to roam.
This is frankly terrifying stuff. I would give it no heed if it weren't for the fact that fallen angels and giants are also mentioned in the Bible itself. Definitely food for thought.