What can I say which has not already been said? Rod Serling was genius! This show was fantastic. Yes there were a few stories that left something to be desired (and did not appear to fit into the meme of the show) on balance I will gladly take the whole of this series to a lot of what is out there today.
Not only did Serling and Co. frighten you at times, they also made you think about your internal and external environment. As Mr. Serling himself stated in the introduction to the show:
"There is a fifth dimension beyond that which is known to man. It is a dimension as vast as space and as timeless as infinity. It is the middle ground between light and shadow, between science and superstition, and it lies between the pit of man's fears and the summit of his knowledge. This is the dimension of imagination. It is an area which we call 'The Twilight Zone'."
This perfect encapsulates the mood of the show.