I find several things puzzling about this. I can't understand why anyone would have voted for it ahead of The Matrix; I also don't understand why Galaxy Quest, which won the Hugo that year (beating The Sixth Sense and The Matrix) and the Nebula the following year, was not on the 1999 Nebula ballot.
The Nebula ballot notoriously included The Uranus Experiment: Part 2, a porn film which includes a 20-second sex scene shot in zero gravity on a parabolic flight. In a spirit of scientific enquiry, I have conducted my own investigation, and I can report that full docking in zero-G was not achieved, at least not in the footage that made it to the screen. (Personally, I think it was an entirely valid nomination, though the film is almost devoid of plot.)
The Sixth Sense has one actor who has returned from a previous Oscar/Hugo/Nebula winner. It is little Haley Joel Osment, playing Cole Sear, the psychic child at the centre of the plot (and in my view the best performance in the movie); five years ago, he was Forrest Gump's son, Young Forrest, in Forrest Gump.
Look, I know that this film is generally considered a masterpiece and won loads of awards. I found it boring and unmemorable. I actually watched it twice within three days, after realising that I had retained almost nothing about it first time round. Without spoiling, there is a major Plot Twist ten minutes from the end, which I spotted eleven minutes from the beginning. Bruce Willis spent an hour and a half working out what I already knew, and it just wasn't very interesting.
I'm putting it right down at the bottom of my list of Hugo and Nebula/winning films, between Curse of the Cat People and Heaven Can Wait. Looking forward rather more to Galaxy Quest, which is next on my list.