This episode is a favourite amongst many fans.
This is the first time we meet Sarek, Spock's father, played my Mark Lenard. He puts in a good performance, and I found him very believable.
Another of the episode's merits is the array of alien races seen. The episode does not include a beam down to a planet, which saved money, so instead more was seemingly spent on the alien designs.
Some of the alien costumes and makeup are quite good (I like the Andorians, even if they do look very '1960s'), but I agree with another reviewer that one race, the Tellarites, has absolutely terrible makeup, and is quite obviously a mask.
The story unfolds well, and has several plots that all affect and hinge on each other. There is the hunt for the assassin, but also Spock giving blood to Sarek, Kirk being badly injured in an attack, and several other strands which all interact with each other. I liked this multi-plot tale, and found it more pleasing than some of the more simple outings of the season.
As good as the episode is, it probably wouldn't make my personal Top 10. Not because it's bad, but simply because there are other stories that I like better. I'm also fonder of stories that involve beaming down to planets.