Island of the Burning Doomed (as the version I have is titled) is a good example of late sixties science fiction, and one of the last good ones to come out of Britain.
The movie is formulaic for those familiar with the genre: Deaths of unknown origin occurring all over a remote island...You'll find the same thing happening in Island of Terror, and a number of other films from the same era.
The story is fairly fast-paced and most of the characters actually have personalities and interact with little of the stoicism and blandness that is often found in science fiction movies. The stars, Christopher Lee and Peter Cushing, partners in so many Hammer films, are somewhat relegated to secondary, but nonetheless enjoyable roles (especially Cushing).
I really enjoyed the movie...the menace is novel enough and kept mysterious enough throughout the majority of the film to keep you guessing, and the performances by the headlining cast are excellent.