I dislike MLP, but I'll try to review this fairly.
The animation was really good, closely mimicking the show. In the start of this, most people were saying this was good because of the animation, or some such. So far, this is a 10.
The voices were okay, I didn't see any flaws in them. These also are closely mimicking the show, which I can't really dislike, as the voices in the show are okay. Pinkie Pie was always slightly annoying and high-pitched to me, however, so I'm going to -1 from the 10.
Parodying this particular show, however, was not a good idea. The ponies just make it seem obnoxious and not funny to me anymore, unlike the old one. I've only watched one episode of the original, and it made me laugh my guts out. This certainly does not have that same effect, it comes very close to infuriation, as a matter of fact. I'm going to -1 from the rating of 9 I said before.
On an unrelated note, I've noticed that all the low rating reviews on the video have been rated useless, except for RicePirate's. This would be considered normal, as they are mostly non-constructive (except for RicePirate's), but all the non-constructive high rated ones are helpful. I think this is because the video is being assaulted by bronies, which find anything that compliments their fandom helpful.
Anyway, this flash has a total of 8 stars, coming from someone who hates MLP.