See I stopped watching Powerpuff Girls about halfway through the first season, not because it wasn't good, and not really because I wasn't the target audience, but because there was nothing really there to interest me and I had better shit to do.
I watched Avatar all the way through multiple times because I thought it was fantastic and interesting despite not being the "target audience" (I'm not sure I believe this though - Avatar was fantastic and interesting because it was more than just a show for kids).
I haven't seen MLP but I suspect it's worth watching if you're:
1. A little girl.
2. Just really into horses.
3. Bored.
4. Friends with any of the above, or with
5. A person who finds small, doe-eyed, pastel-colored horses sexually attractive.
In the interest of full disclosure: if a tiny blue mare named Sparklebutt batted her lashes, assured me I wouldn't get a disease and lifted her tail... why the fuck not, I've probably thought about worse.
Like Blossom in high-heeled leather boots and nothing else.