It took me a month of lethargy to get through 88 episodes of Sailor Moon. I personally think the Black Moon arc is one of the strongest parts of the anime. First, it takes a character through an entire arc of development. While all the characters in Sailor Moon grow and evolve(that's one of the selling points), Chibi Usa's development here is particularly note worthy. She goes from a scared little girl who acts out because she is in a truly frightening situation, to being able to face up to her mistakes and fix them, even if she has to become the (really cool looking) embodiment of darkness beforehand. I mean, imagine this. You're 5 or 6 years old. For all you know, your mom is dead, and your dad gravely injured, and it's all your fault. This isn't going to be sunshine and vanilla pancakes, and you're not going to act like miss sunshine.
Second, the villains are engaging and human. I like Sailor Moon's formula of having some villains that can be convinced, and villains that have real motivations, besides ohohohohoh, we're evil. Sure, Wiseman/Dark Phantom isn't exactly Mr. Motives, but the rest of the clan make up for it. Dimande's desire for revenge, Sapphir's brotherly love, and Esemeraude's love for Dimande are all solid reasons that they fall for the trickery of the Wise Man. This is a pretty good addition to a children's show. People don't just wake up, and are evil, but usually have ways to justify themselves. Also, I like the fact that they included some conversation, some idea that people with differences could live together, which encourages a more peaceful viewpoint.
Third, it also helps flesh out the 'back story' of Sailor Moon, by showing the future. This makes the imaginary world of the cartoon richer and deeper.
Forth, even the filler episodes are fun. It's hard to keep a good momentum going when you use filler, but it held up tolerably well.