I felt Sybok was the most effective thing about the movie.
The actor himself was fine but I just could not get behind the idea that Spock suddenly had an emotional 1/2 Vulcan brother.
I never understood why some people were against Spock having a half brother. Before TOS episode "Amok Time" we did not know about Spock's wife T'Pring and yet people don't complain about that, they accept it. Before this episode Spock never mentioned having a wife. So Spock having a half brother was not a problem with ST5 for me. I remember Majel Barret-Roddenberry complaining about us not knowing Spock had a half brother at a Star Trek Convention a year or two before the film was released.
We also did not know about Kirk having a brother, Sam, until he mentioned it in an episode of TOS ("What Are Little Girls Made Of?"). Later, we see his brother dead in TOS episode "Operation - Annihilate!". People never complained about this either, they accepted it. So why is it acceptable for Kirk to suddenly have brother and it not acceptable for Spock to have a half brother?
Navigator NCC-2120 USS Entente
Stokely: In an insane society, the sane man must appear insane.
Harvey Holroyd: Where'd you get that?
Stokely: Star Trek. [leaves the room]
Harvey Holroyd: [to himself] God, I miss that show. (Source: "Serial", 1980.