I'm going to guess William Shatner found the concept of this episode interesting as it bears a strong resemblance to the one Trek film he did direct, 'The Final Frontier'.
I see some people quite like this episode and it is an interesting curio, obviously influenced by the late 60s flower power era, but I also find it cringeworthy too. An obvious film set for the surface of the planet Eden, poor prosthetics on Dr Sevrin, bad wigs and costumes plus dreadful dialogue are among a myriad of reasons it is not one of my favourite episodes. On of the few reasons to watch it is to see a pre-fame Charles Napier as one of the gang of 'space hippies', fifteen years before he sent Rambo off into the Jungle to rescue POW's.
Series three of TOS was renowned for producing some clunkers and i'm afraid I have to put this one in the five worst episodes of TOS category.