As a huge Rarity fan, I'm inclined to like this episode more since the focus is on her, but I think even non-Rarity fans can enjoy this episode. The setup is rather simple; Rarity travels to Canterlot and gets wrapped up in social activities there, which start to interfere with plans she has made with her friends. Rarity doesn't want to snub her friends, but cannot resist the allure of Canterlot elite status.
The episode centers around the mental wall Rarity has erected between her friends (the Mane Six) and her new connections in Canterlot. As it goes on Rarity ends up procrastinating on a promise she made to Twilight in order to attend events in the city, and eventually lies about not being able to attend Twilight's birthday party. While this is going on she tries to conceal her Ponyville roots so she can keep her upper-class status, which works out well until Twilight moves the birthday party to Canterlot so Rarity can attend... right next to the garden party Rarity was going to attend instead.
Some people complain the lies go unpunished in this episode, which is partially true; although they aren't individually addressed, they all contribute to the larger problem Rarity experiences in trying to keep her roots a secret. This is appropriate enough, since the lies were intended to satisfy both the Canterlot ponies and her friends while keeping them as separate worlds, yet ultimately the deceit makes such a task impossible. By the end of the episode Rarity's friends have completely embarrassed her in front of the entire garden party, which I find a rather fitting karmic ending that goes well with the Aesop.
I did take issue with Twilight's idealism about Rarity's present (a dress for Twilight's birthday) and reason for attending the garden party. I can see how SHE would be happy with such a plain design, but Rarity's designs are usually about as subtle as a slap to the face. We've also seen an elite streak in Rarity that might imply she wants to accompany the Canterlot ponies more than her friends, yet Twilight assumes (wrongly) that Rarity sneaks to the garden party for business reasons. These are small complaints though, and I might be assuming too much cynicism from Twilight.
In short, I liked this episode very much. It isn't my favorite of the second season (Sisterhooves Social) but certainly a worthwhile watch.