A 6. It isn't bad, but it has some things that keep it about average. It's TOS and characters and a universe I really like. I also like the lighting and moody atmosphere.
The women's transformation, particularly Eve, is something that can't be taken too literally even though that is the tendency for viewers when watching film or television. Beyond perhaps affecting a person's complexion and a few other things the Venus drug could well be causing the taker to put out powerful pheromones that affects the perception of others. That's the only explanation that really works for me. Later we will see Enterprise women who are easily as beautiful or more so than Mudd's three so for the Enterprise men to be so taken with Mudd's women (when under the drug's influence) suggests there's something else going on. In real life when we become infatuated or focused on someone we tend to see them to largely the exclusion of others, at least in the beginning. And we know there are definite biochemical changes happening to both men and women when we're "in love." So it could well be the Venus drug affects more than just the user.
In somewhat similar fashion is the Salt Vampire actually changing it's form or does it somehow affect the perceptions of others? How else to explain that different people can see different things at the same time? Kirk, McCoy and Darnell each saw "Nancy Crater" as someone different all at the same time. Later the creature learns to be more consistent in its appearance around Kirk and McCoy and others so it can influence what others are seeing so as not to cause confusion.
The idea of the dolled up women being "wiving settlers" isn't that far out as today there are any number of means for men of western countries to communicate with women from other countries interested in finding a mate abroad. And often these women (and men) can be made to seem more than what they actually are.
Harry Mudd establishes that TOS' future isn't perfect and ideal since there is obviously still crime only now taken to an interstellar scale. This episode also shows the role of Enterprise's role as policeman as well as explorer and soldier.
Harry Mudd explained that the Venus drug gives the user more of what he or she already has - their sexual qualities are enhanced
. We never saw what the drug did to men, but Harry said it would make them more masculine, and presumably the women would react to them similarly to how the men reacted to the three women. It's not only a matter of how they look - it's pheromones, and every other sort of biochemical stimulus involved in sexual attraction.
What I found ridiculous was that at the end, Eve was able to physically transform herself via a placebo, and not the real drug.
About the Salt Vampire: It had to have been at slightly empathic, if not somewhat telepathic. Otherwise, how could it have transformed itself into the "perfect man" that Uhura would have found attractive - one that speaks Swahili, no less?