We finally have a good Spike episode! It feels is though so much time has passed since Spike was a central protagonist and not totally out of character.
The moral of the story fit Spike quite well given his predilection from gems and his previous brush with uncontrolled greed. Spike wasn't so much a bumbling imbecile (I'm looking at you, "Spike at Your Service") but rather blinded by his own shortsighted desires. It's difficult to imagine a more straightforward premise: consider that the whole episode literally revolved around Spike wanting to have his cake and eat it, too. Yet the narrative felt loyal to the characters and avoided overplaying non-sequitur gags. Spike should not be a blanket excuse for poor writing covered up by visual jokes.
Speaking of sight gags and such, "Just for Sidekicks" did a nice job of incorporating visual humor---a must since the pets can only really interact through their non-verbal behavior. The physical comedy did not come at the cost of other aspects of the narrative. So far it seems only "Sleepless in Ponyville" has truly achieved the balance between physical jokes and character interactions. This is the first episode in a while that made me laugh aloud, with the animal dryer gag giving me the biggest laugh. I haven't laughed so many times since "Ponyville Confidential", one of my favorite episodes in the series.
The mass involvement of the pets resulted in an interesting combination and exploration of their personalities. Angel and Opal's behavior is fairly evident through their high volume of screentime versus the other pets. But now we know that Owlowiscious is the most "responsible" of the Mane Six's pets; Gummy is either immensely enigmatic or just a few bricks shy of a full load; Tank is eager yet somewhat misguided and/or inexperienced; and Winona is as loyal as her owner. So each of the pets reflect their owners in various ways. I'm glad the episode did not try to over elaborate on their character traits too greatly; the pets were best used as supporting characters (save for Angel who, unlike the others, appears to be at least quasi-sapient). And that is exactly how it played out.
Overall I'm very pleased with the episode. I am only left wondering if this episode was intended to take place during the next episode when the Mane Six are slated to go to the Crystal Empire. If such is the case, it might be funny to see some of the antics from "Just for Sidekicks" appear in the background.