3.5
"The Zap Gun" is not one of PKD's best, but is still a fun read. I've read a number of Dick's works and would rate "Zap Gun" towards the middle. It has great ideas, some of which are pretty relevant to today's world, but there are some plot lines that could have been either more developed, or better incorporated into the rest of the story.
Overall, while "Zap Gun" could have been even better, it's still good and most PDK fans will probably enjoy it.