This episode is not based on any chapter from the manga, which in a sense makes it a filler episode. However, it certainly isn't filler in the same sense as the anime's many "Monster of the Week" episodes. It contains crucial character development, and is certainly an episode not to be missed.

In the manga and anime, Ami was always mildly interesting, but her flaws and insecurities were never very plausible, and in the anime they were always brought up for token Ami-related filler episodes and then forgotten about. On the other hand, PGSM has made Ami into a character with depth. She has flaws and insecurities that are easy to relate to, and she isn't by any means a static character. The issue of Ami's hesitance in making friends has been building since episode 2, and here's it's come to a head but not a conclusion. Until now Ami has never received the benefit of a continuing thread of character development like that. This is quite a revelation.

Not only that, but it was done really well. All sorts of interesting techniques were used to get us inside Ami's head and into her world, from having her narrate the recap, to the "day in the life" scene at the beginning, to various inspired lines and interesting camera angles, and - of course - her hilarious fantasy about Usagi and the gun. In particular, I loved the way the glasses were a psychological barrier to her making friends, and a metaphor for her insecurities. The ending of the episode is reassuring without being unrealistic or heavy-handed: It shows that Ami is overcoming her fear but staying true to herself. Building friendships can take time, and putting on an act won't change that.

Isn't it interesting that PGSM makes this obvious truth as clear as can be, and yet in the manga and anime we all just took it for granted that the girls became friends within a single episode?

We knew before PGSM started that it would focus more on the girls' relationships with each other than the manga and anime had. That's definitely the case, and I'm extremely impressed. This episode is a fairly big departure from the manga and anime, but in this case that's a very, very good thing.

On a different note, the final scene of the episode is when Makoto appears for the first time. This scene is reminiscent of the last few panels of Act 4 of the manga.