The main problem is that this episode really isn't about anything significant. At worst, Garth is going to kill three people (where is the rest of the asylum staff?) and then get stunned when he tries to leave. Most of the episode takes place in bland interior sets.
It's a credit to Steve Inhat (one of those 60s "Hey, I know that face!" guys) that he invests the role of Garth with dignity, despite the stereotypical insanity the character is saddled with. Even then, Inhat at least puts some enthusiasm and menace into otherwise inane dialogue.
Yvonne Craig does a convincing job as a sexy insane woman.
The idea of a sign/countersign is a good example of common sense, despite the fact that they forget it in "Turnabout Intruder." Whoops.
Garth's shapeshifting ability seems conveniently stapled on for a show already burdened with twins and lookalikes. So he was taught cellular regeneration techniques and was smart enough to become a full-fledged shapeshifter as a result? Ummm, okay.
The shapeshifting, like several other elements in the script, comes across as contrivance. A planetary-wide forcefield, for instance. Ummm, okay, first of all, why? Just force field the asylum. And what the heck powers it?
Spock deciding to let the Kirks fight it out is another contrivance. Although his line about shapeshifting taking up energy neatly dovetails into Odo needing his rest periods in DS9.
Otherwise the episode comes across as low-budget third season material. Everything here is recycled, from Andorian costumes to neutralizer chairs to Garth's costume.
Overall there's some clever dialogue and Ihnat's performance (and Keye Luke manages not to embarrass himself). But definitely not a must-see.