If “being cool” is all you care about, maybe you should go make some new cool friends, somewhere else. – Rainbow Dash

Rainbow Dash is surprised to find a fellow prankster in the usually annoying Pinkie Pie when she is enlisted to help prank Spike by scaring him into getting hiccups. This leads to a strengthening of their friendship, as they spend the entire day pranking the rest of their friends. The pranks are all in good fun and pretty harmless, leaving even their victims smiling or chuckling. Only Fluttershy escapes their attentions, on account of her being too sensitive to prank.

I hope that doesn't come into play later on or anything...

The next day, Pinkie means to carry on the same as before, but when she arrives at Rainbow's cloud house, she is instead met by Gilda the Griffin. After Rainbow introduces them, Pinkie invites Gilda to join them in their pranks. Gilda is more interested in spending some flight time alone with Rainbow, and they leave behind a rather dejected Pinkie Pie.

After a while, Pinkie Pie starts making a nuisance of herself by trying to butt in, which annoys Gilda. Pinkie uses a trampoline, so Gilda flies higher. Pinkie then uses a bunch of balloons, so Gilda secretly pops them. Then that dashing young filly on the flying machine turns up again, and Gilda sends her into a spin which crashes her.

Finally getting the message that she is not wanted, Pinkie vents to Twilight, who suggests maybe Pinkie is being jealous of Gilda and is finding excuses not to like her. Pinkie leaves in a squeaky huff, but soon after wonders if Twilight might not have a point after all. However, seeing Gilda scare Granny Smith, steal an apple and (horror of horrors) ROAR AT FLUTTERSHY, convinces Pinkie she was right all along. Now it's time to solve a problem named Gilda... PINKIE PIE STYLE!

Which is "throw a party". During the party in honour of the new guest, it seems very much like things have been set up as a humiliation conga for Gilda, as prank after prank finds her. Finally blowing up, she rails at the rest of the ponies, accuses Pinkie of trying to make a fool of her with "lame" pranks, and tells Rainbow that the two of them should bail on the pathetic pony party. Rainbow isn't having it and tells Gilda that she was the one who set up the "lame" pranks, and it was only bad luck Gilda triggered them all. Then she gives an ultimatum... treat my new friends well, or go find some other "cool" friends. Gilda seems to think it over, but then accuses Rainbow of being a "flip-flop" and storms out. Twilight apologizes to Pinkie for being wrong about her and Gilda, and sends a letter to Princess Celestia with the moral: "You can't control who your friends hang out with, but you can control how you react to it."

Character Study:
Pinkie wasn't very well behaved in this episode. She came across as a bit clingy, and pushed herself in where she wasn't welcome. Although Rainbow didn't mind, Gilda clearly did, so Pinkie seems a bit insensitive.

That said, Twilight did make a mistake in suggesting Pinkie was in the wrong. First of all, she knew very little about the situation, yet acted as if she did. Secondly, most of the time she was reading a book and paying very little actual attention to what Pinkie was saying. This is emphasized by the episode's cold open, where again Twilight is reading and not paying attention to what Pinkie is saying. She even heaves a visible sigh of relief when Pinkie goes running after Rainbow.

As for Gilda herself, while she is pretty lame by trying to act cool all the time, she does seem to genuinely care for Rainbow, even being willing to do a song and dance routine she clearly finds unenjoyable. Sure, she doesn't behave really well, but I think many fans dislike her to a disproportionate degree simply because she mistreated Fluttershy. Even her outburst at the party is in a way understandable. Pinkie greeted her and then immediately pranked her with a hoof buzzer. Why shouldn't she assume that Pinkie was responsible for the rest of the pranks too, or that they were deliberately targeting her? On the other hoof, she did act arrogantly...

Rarity: Can I go first? Can I have the purple tail?

Gilda: NO! You already have a purple tail!

...so it's not entirely unjustified when Rainbow calls her out on her mistreatment of others.

Moral:
Sometimes you have to share your friends with other people, even when you don't like them very much. A decent lesson for kids to learn. Not much else to say there, it's a decent moral, but nothing ground-breaking.

Pros: Some of the pranks are pretty amusing, and we see that Twilight is not so worried about staying in Princess Celestia's good books that she can't prank her teacher. Introduction of the griffin species and more information about Rainbow's early days is good too.

Plus this conversation:

Fluttershy: Um, Pinkie Pie, about this party for Gilda... um, do you really think it's a good idea... I mean...
Pinkie Pie: Don't worry your pretty little head about Mean Old Gilda... your Auntie Pinkie Pie's got it all taken care of.
Fluttershy (Annoyed): I'm a year older than you.

Cons: Making Gilda roar at Fluttershy just to make it clear that she is "bad". Given the blind devotion of some fans to the shy pink and yellow pony, it seems a bit manipulative.

Gilda's way of speaking is a bit annoying.

Overall impression:
A decent episode, with some minor problems and an unlikeable antagonist, but well put together.

Final Rating

5 – Celestia Rank: A great episode. It will be re-watched frequently.
4 – Luna Rank: A good episode, but with one or two problems that prevent it from being great.
3 – Spike Rank: An average episode. Positives and negatives are balanced.
2 – Discord Rank: Worth watching once. After that, turn it to stone and put it in the garden.
1 – Nightmare Moon Rank: Send it to the moon!

Stay sunny side up!