Ashley Gable and Thomas Swyden's clever script, based on a story by series creator Joss Whedon, is jam-packed with clever dialogue. Other writers would do well to use it as a model.
It also has a carefully-polished plot, making good use of the theme of alienation. That said, it's not the most original story, and it can be no coincidence that Willow is wearing a Scooby Doo T-shirt is some scenes. All the series needs is a pooch and a van, and Hanna-Barbera would be consulting their lawyers!
This episode thrusts Cordelia into the spotlight, and Charisma Carpenter proves to be more than up to the challenge. It's a pivotal episode for her character, redefining her relationship with Buffy and the gang, and mellowing her haughty attitude.
The scene where she opens her heart to Buffy on the subject of her isolation at the eye of the popularity storm, is very poignant. By the end of the episode, Cordelia is practically one of the Slayerettes, even though there is still some animosity.
It also brings a new dimension to Angel's relationship with Giles, as he retrieves the book about Slayer lore that will play such an important part in the series' finale.
The explanation for Marcie's invisibility (the result of people's indifference combined with energy from the Hellmouth) is particularly inventive, and deftly bypasses the obvious potions and scientific experiments.