It was nice to see a Spike- and Rarity-centered episode. Spike's a great character, and I wish he'd feature more often; I really like him, his friendliness, his encouragement and support he offers his friends, and his dry wit. He really is an observational comedian — and a fun character in general. The stairs to the spellbook crumbling underneath him while he's reading it, oblivious to what's happening, was a great example of a Spike gag.
Rarity's also great and (at times) a bit underappreciated. Still as prone to dramatics as ever as well, and thanks to that we now know that she looks rather adorable with runny mascara. (Also: she wears mascara in the first place. Did any kind of make-up ever feature in MLP:FiM yet?) The stuffing-herself-with-ice-cream bit I was briefly unsure about in light of the lesson it would teach, but then again the whole thing was clearly and obviously played for laughs, not straight.
It was interesting to see Rarity's own magic being overtaken and her sound judgement clouded by the book's dark magic. (Which, I noted, was incidentally the same color as Chrysalis's magic; apparently a greenish glow indicates sinister sorcery.) It surprised me that Spike didn't notice this, but him going along with her newfound sense of creative mission and supporting her, finding himself drawn deeper and deeper into events he realized he couldn't control worked very well. By the time realization sank in, it was too late, and worse, he'd already given his promise, so he was really caught between a rock and a hard place. That he was willing to put the greater good ahead of his own desires and risk losing Rarity's friendship shows that he's a fairly mature character already.
Owloysius featured extensively in the episode as well, mostly in the role of a monosyllabic Cassandra. He seemed to know bad things would happen all along — why/how, though? Well, I'll grant that spellbooks locked in secret alcoves and festooned with spikes are rarely good news.
The resolution of the story was fairly quick, perhaps a bit too quick: Spike really just had to say "it's awful". When Rarity turned to ask "what did you just say?", I thought there'd be some complications, but no, the spell's dark magic just left her and dissipated. OTOH, it was just a spell(book), not an intelligent agent that they were up against. In fact, I'd say it wasn't even evil in any usual sense of the word: a spell just is, after all. That said, why anypony felt the need to create, much less preserve such a spell is another good question.
The rest of the Mane Six really just had bit parts (except for Twilight); Rainbow looked quite good in her dress, though, and Twily was adorable all ruffled and tired in the end. Spike's final line made me burst out laughing, too.
Speaking of Spike again, I hope he won't get a belly-ache from that book... but dragons are made of stern stuff, I'm sure. When you can melt metal padlocks with a single breath of flame, you'd better be.
There were quite a few background ponies as well; Berry Punch, Vinyl Scratch, Amethyst Star, Dinky,
Also, speaking of the princesses, Twilight's really getting some recognition now, isn't she? She's the pony to go to when things go south, and she's one of those who'll clean up the big mess because nopony else can.
Oh yeah, and there was the puppeteer: Claude, apparently, if the credits are any indication. Going by his model, I'd guess he might be Snips's father as well.
Best face for the episode: Rarity's smug face when she said "a designer never reveals her secrets".
Also: Rariot. I'm totally gonna use that word from now on.
Elsewhere: discussion in broniesaremagic. Episode follow-up on EqD.