Every episode was a self-contained story, so it's got some good episodes and some bad. Some of them have become so iconic that they're part of the US cultural landscape to this day, such as "To Serve Man," "Eye of the Beholder," "Time Enough at Last," and many others.
Before he was on Star Trek, William Shatner was in an episode of the Twilight Zone where he played a guy who, on an airplane flight, saw something on the plane's wing that was damaging the craft. No one believed him, of course, but it was really there (as revealed by the damage at episode's end).
Fun fact - that same episode was remade years later as part of a Twilight Zone movie, with John Lithgow in Shatner's role. The whole thing eventually got a nod in an episode of Third Rock from the Sun.