It's like "Destroyer" is stuck in a state of half-development. The basic storyline would be okay, but the script gives Conan a coterie of unneccessary sidekicks (as if Conan were not a strong enough character to carry a movie by himself). Grace Jones is interesting to look at but gets tiresome VERY fast; the cowardly sidekick gets more irritating by the second; Mako wears this expression like he's only in this one for the money. Olivia d'Abo is just plain miscast: it's like she's a marker that should read "Insert More Interesting Character in Second Draft." Arnold Schwarzenegger does a competent job as Conan, but we don't see the raw power and brute force of the first movie. There is a thin line between competence and complacency, between an actor and a star; it took Roger Moore six years to get to that point in the James Bond series, while Schwarzenegger reaches the same point with Conan in two.
I think part of the problem is whether or not the Conan concept is flexible enough to become family fare, and my answer is "no." Fans of Robert E. Howard's Conan work know his Cimmerian is a very rough-edged hero, a cunning, hard-drinking, hard-fighting man, and the first Conan movie captured that essence. To change him into the simple-minded do-gooder of "Conan the Destroyer" is to destroy what Conan is all about, and the movie suffers for it.
Is "Conan the Destroyer" worth watching? Yes. For someone who knows absolutely nothing about Conan, I suppose it can be a harmless sword-and-sorcery popcorn flick. For Conan fans, it's kind of like "Legend of the Lone Ranger" for Lone Ranger fans: an irresistible example of just how badly Hollywood can treat your favorite character.
I just hope that if there is ever another Conan movie (not likely, thanks to "Destroyer"), the producers will concentrate on following the style of "Conan the Barbarian" and just ignore this one.