I actually met him once and was almost flattened by his ego. OTOH, Ray Bradbury and Jerry Pournelle (a bit loud) were pretty good guys. Bradbury was very old but still pretty sharp.
If you read the story about HOW the actual script came to be for The City on the Edge of Forever, you will find out who REALLY wrote the screenplay. But the way that the rules in Hollywood work, Ellison “wrote” the show.
I was at a club meeting when I overheard one author observe that Harlan Ellison's ego was so massive, it had collapsed into itself and became a black hole, sucking the light out of any room he entered. Harlan laughed about it as the lights in the room suddenly went out. The electrical problem was quickly fixed, but has always stuck in my mind as an example.
The first time I met him was nearly 40 years ago when I was hauled to a Christmas pool party, and I was very much a brat to someone who had drunk too much. Being bitter from being at this party on my birthday, I took it as a challenge when my mother grabbed my arm and pointed me at Harlan, telling me that if I didn't have anything nice to say, I should go say it to him.
Took another six years before I fully understood some of his replies to my eight year old taunts.