Generally (but not always), I'm not as keen on completely Enterprise-bound stories. But I really like this story.
As with many stories from the Original Series, it very much serves as a moral fable, and in its own way is quite poetic.
Being an early episode, there are a couple of slightly out-of-character moments with Spock, especially during Uhura's song in the rec room when he can be seen to break a smile. Personally, I like to explain this away rather on the same level as Data experiment with different moods in 'The Next Generation' – Spock was just "trying it out" (!).
Charlie is well played by Robert Walker, Jr., giving a disturbing and sad performance in equal measure.
There are some good moments as Kirk tries to teach young Charlie how to behave properly, and later when Charlie starts to use his mental powers to take out his anger on people. One particularly notable moment comes as he wipes away the face of a poor young woman.
The resolution of the story is quite eerie and haunting, although I did feel that it played out a bit too long and the pacing could have been tightened up.
All-in-all, although it probably wouldn't make my Top 10 favourite episodes, I like this episode.