Like many third-season episodes, this one tries to be rather experimental. It's very much a horror/possession story: The Zetars (Zetarians?) aren't really aliens as much as a demonic "I am Legion" type of force intent on taking over the life and body of a human being for their own purposes.
Despite this fact it comes from Shari Lewis of "Lambchop" fame, it features some definitely creepy scenes: the deaths on Memory Alpha, Romaine's possession episodes, the scenes of the possessed technician/Romaine changing face-color and making that weird garbled noise (a recycle from the aliens at the end of "Catspaw"?), and the grotesque form of death aliens bring. And we get another alien race, powerful, mildly apologetic, but with iron-willed determination to save themselves. Again, we get another "alien" alien in the third season. And the idea of a Federation library.
Unfortunately, how it plays out is kind of flawed. James Doohan purportedly wasn't thrilled with the romance, and it's not surprising. Despite his best efforts (and some really awkward dialogue), there's not any chemistry between the two romantic leads. Jan Shutan does the horror elements well, and the character's late-in-the game "I will not be taken over" determination, and she's not conventionally drop-dead gorgeous. However, she doesn't really sell her side of the romance angle.
There's also that excruciating log entry by Kirk at the beginning ("When a man of Scotty's years..." wow, way to rub it in), and the whole "high pressure can kill the aliens" angle that comes in from left field.
So I'd give it around a 7 for general creepiness and alien alienness, but it doesn't really make it into the top tier. Just cruising along in the middle of the middle range.