I've watched a few episodes of the show.  I never really thought about it much. I thought it was kind of weird, but I was never a hater.  People like what they like.  I have my fandoms, and they have theirs.
But the more I heard about, the more curious I got, and my friend who's a brony directed me to a documentary about bronies.  In there was a guy who used to be an artist, went off to fight in Iraq or Afghanistan, and when he came back, found that he just didn't find any joy in anything anymore.  Not in his art, and not in anything in his life.
Then he started watching Friendship is Magic, and it affected him ... positively.  He started drawing again (pony fan art, mostly), and started functioning again.
I figured, "OK, THAT is a powerful testimonial".  So I had to watch a few episodes.  I didn't have a transformative experience, but I gotta say that it is a good, cute, fun show.  And I can see the appeal.  If I had kids, I would definitely endorse them watching this show.  And I still watch an occasional episode, when I want some cheering up.