The actor himself was fine but I just could not get behind the idea that Spock suddenly had an emotional 1/2 Vulcan brother.
What? Sybok was a full Vulcan. We were told in the film that he was the son of Sarek and a "Vulcan princess." And since we know that Vulcans are betrothed as children, it's reasonable that Sarek had a Vulcan wife before he married Amanda.
And Sybok's rogue philosophy helps explain why Sarek disowned Spock for going into Starfleet, why they went 18 years without speaking as father and son. Maybe Sarek was afraid that Spock was following the same path that Sybok had and would bring similar shame to the family.