For me it's a "4." Not one of Star Trek
's finer outings. There's more than a little sexism to be found here, and the magic drug belongs in a children's tale, not a series that was allegedly targeting an adult audience.
On the technical side, the episode suffers from editorial sloppiness. For example, the editors were forced to use a shot of McCoy in sickbay as a close-up for the first transporter room scene. Not only do the backgrounds not match, but McCoy is in two different costumes. This apparently happened because the director, Harvey Hart, decided to "cut in camera." That's fine if your Alfred Hitchcock, but it causes problems when you're a director-for-hire on episodic television.
On the other hand, Hart's sense of composition is quite good, but since it came at the price of going a day over schedule, it's not hard to see why he wasn't asked back.