I have mixed feelings about this one. I've made a few comments on the page but I'll speak of it in full detail here.
On one hoof, the zombie scene was pretty cool and creepy. It would have been an interesting twist that it was all a prank, if the synopsis hadn't spoiled that ahead of time.
On the other... This episode wasn't a show-killer for me or even a season-killer, but I didn't enjoy it as much as I thought I would.
It all comes down to Rainbow Dash's portrayal in this episode.
First, she is quite out of character in this one. Season 1 showed the darker side of her personality (her being impatient with Fluttershy, cheating to win the competitions, and calling Rarity a laughingstock), but before any of those episodes she agreed with Pinkie Pie not to prank Fluttershy when Pinkie explained how sensitive she was. Furthermore, later seasons show her to have become extremely caring to Fluttershy and Scootaloo, and nearly giving up becoming a Wonderbolt when she thinks they reward reckless behavior.
Yet here, in contrast to Griffon the Brush Off, well, we've seen how it goes, none of the ponies find it amusing and she gets out of hand, refusing to listen when Pinkie asks her to stop, and the first pony she goes after - before she even tried to "put more effort into her pranks" is Fluttershy. Also Pinkie didn't show any signs of disapproval, unlike in Griffon the Brush Off.
Second, this episode probably destroyed what little remained of Rainbow Dash's reputation within the fandom. Her last episode, Stranger Than Fan Fiction, was awesome, and she was so awesome and cute in that episode. Yet this episode cancels it out. I've seen many comments (and not just on here) saying Rainbow Dash is now worst pony, and has been removed from favorites lists, if nothing else than for the Fluttershy thing.
Yes, it was an out-of-character episode for Rainbow Dash, and Fluttershy herself was far worse in Putting Your Hoof Down, and Rainbow Dash learned the friendship lesson in the end, so next episode she appears she'll probably stay the awesome pony we've always known, and nopony else's opinions on Rainbow Dash will influence my own, but still...
Edit: I don't dislike Rainbow Dash yet, but she is no longer tied with Fluttershy as my favorite pony. She is now moved down to second. If she gets an awesome and/or cute portrayal in a later episode (though Buckball Season sounds ominous, but hopefully Top Bolt will be good), she'll go back up to Fluttershy's level.