Whenever someone tells you that a Bollywood film is science fiction, never believe them. PK is no exception. Like all Indian movies claiming to be science fiction, the sci-fi element is basically loosely used for the setup, and not really explored. PK is an alien who comes from another planet, gets stranded on earth, and largely spends the whole movie trying to "find God".
Not that this makes it a bad movie. PK was a bad movie on its own, without the sci-fi elements. PK is very preachy, showing the hypocrisy of various religions, but it fails to entertain, or even give a coherent plot.
Am I surprised or angry that this was not a very good film? No, that's par for the course for Bollywood. I'm more upset that it received positive reviews from critics. Indian film critics are the real hypocrites here. They judge movies based simply on stars and budget. If this was not an Aamir Khan film, then it would have been panned.
My recommendation: skip this film, there's nothing to see here.