"The Empath" is a mixed episode, and seems to be a 'love it or loathe it' story. But it has some very good concepts included, even if they weren't all executed perfectly.
For someone who has no dialogue, Kathryn Hays gives a very intriguing performance as mute empath Gem. I was really drawn to the character, and found her very memorable.
In some ways, I felt this to be a poor man's version of "The Cage" (the original, unaired pilot for the series). First, the aliens' visual appearance struck me as similar – the long cloaks, the elongated heads – and the similarities followed through to much of the plot, with their 'experiments' and illusions, and Kirk and co. being their 'subjects'.
The plot is mixed but mostly I found it to be quite good. One thing I did feel though, was that it was a bit dragged out in a couple of places to fill the running time.
One of the things that many people don't like about this episode is the sets – or rather, lack of. Indeed, the vast majority of the story is filmed in a large, dark hall, with no visible walls or anything. I suspect that this might just as easily been down to the third season's drastically reduced budget as it was to any sort of artistic intentions.
One good thing about the episode is the in-episode score, which suits the story perfectly, particularly that relating to Gem.
The late DeForest Kelley (Dr. McCoy) mentioned this as (one of) his personal favourite episode(s), and its nice to see it get such a recognition, as it is an often overlooked and underrated instalment.
I totally disagree with those who say this is the worst episode of the series – there are some far worse offerings. And this is coming from someone who has sat through the terrible "Spock's Brain"!
All-in-all, although far from perfect, this is an intriguing third season episode, and doesn't deserve to be so underrated.