I watched this because I'm a buffy fan and wanted to see Antony Stewart Head. But it's REALLY good. Everything is sung, it's an opera, or musical if you prefer. Very dark, originally done on stage, it's a great mix of dark and gloomy and upbeat action. The central characters are believable, while being surrounded by morbid caricatures of people's worst impulses. Basically, this is a live action cartoon.
If you liked the Buffy Musical episode and other Joss Whedon work (which this isn't), you'll probably like this a lot. Oh, and check out Dr. Horrible's Sing Along Blog for more dark humor and song. You can see it online for free, or get the extras on DVD.