The idea of a Spike-centric episode, with the moral being that Spike, despite being neither an Element of Harmony nor a pony in the first place, is still as helpful and appreciated and as much of a friend as all the Mane Six — that's a good idea, in principle; Spike gets treated like the proverbial fifth wheel all too often, frequently not appearing in fan art depicting the Mane Six etc., so an episode to emphasize that he is part of the team, as it were... that's something I can get behind, yes.
But this story? Spike reads a comic book, and he and the Mane Six somehow magically get sucked into said comic and dumped into its story, transformed into superheros, only able to escape again after defeating the comic's fictional villain? Come on. That's utterly crazy, doesn't make any sort of sense, and (worse) really isn't even all that much fun. If I were interested in a stereotypical (if tongue-in-cheek, but that's run-of-the-mill these days as well) superhero show, I'd watch something else, not MLP:FiM. The reason I'm watching this show is that it's better than that: that instead of "it's time for our weekly crazy adventure, at the end of which everything will be exactly like it was before again", it offers intriguing stories, continuity, character depth and development, slice-of-life as well as high drama, and an original, fascinating, believable, internally consistent, well-thought out and just plain beautiful world.
Well: that it usually offers that, because this week, I didn't see much of any of that. I don't want to say the episode couldn't have been worse, but it was the first one (ever!) where I felt that I could've just as well done without watching it, and I'll file it under "I watched it once for the sake of completeness, now let's not speak of it again". But I suppose you can't win all the time, and season 4 has been pretty exciting up to now, so I can forgive one stinker. Next week's episode will make up for it again... I hope.
I did like Twilight's mask, though.
Elsewhere: discussion in broniesaremagic; episode follow-up on EqD.