The Empath has to be one of the most polarized episodes of not just Star Trek TOS, but possibly the whole franchise!
There are people who rank this as high as City on the Edge of Forever, Balance of Terror, and the Doomsday Machine.
And then there are people like me why Star Trek only has 78 episodes because of how horrible this is.
I will explain my reasoning:
I can deal with a minimal background,
I can deal with that it was not an action packed or Enterprise heavy episode (like The Tholian Web),
I totally respect that McCoy sacrificed himself putting him near death to save Kirk and Spock.
What I can not deal with (even after 40 years) is the fact the Vians get away with killing Linke and Ozaba the scientists and very nearly do the same to the landing party after capturing them just to test Gem's healing abilities-and no sort of retribution comes of the Vians except a speech from Kirk on what they are doing is wrong (especially when 2 episodes prior in Plato's Stepchildren, Kirk was able to get revenge on Parmen after just humiliating he and Spock through the whole episode).
If the Vians got trapped in a forcefield forever and Gem was taken back to the Enterprise for treatment at a starbase it would have been one thing, but to not have them face any punishment is just unwatchable.
BTW: I understand this is just my opinion for the ones that do like the Empath, because we all as Trekkies have different tastes.
Live long and prosper Trekkies!