Sadly I haven't seen the english version (So I can't give any pros nor cons about the english voicecast) but from what I've seen in the japanese version... Incredible. I really like mini series like these, with only 24 episodes, so that there's praticaly no fillers whatsoever to distract us from the main plot and story.
The main character is Rosette Christopher, a cheery 16-year old who's eager to find his younger brother again. Ever since her brother was taken away by a being she reffers to as "Aion", Rosette insisted to join the Magdalene Order to become a nun. The Magdalene Order is an organisation of which consists people who work day and night to rid of the evil that surrounds the cities of America in the late 1900's. Demons, as they call it. However, the new technology of those days build great arsenal power to the organisation. Guns with bullets filled with Holy Water, and more. Anyways, Rosette always fights demonic monsters with the help of her companion, Chrno. He's a great friend and very loyal to Rosette, even though when he does smart remarks, she always goes a little rough on him. But not too rough, just to kid around. There's a strange bond between Chrno and Rosette, and for that... you'll just have to find out within the episodes, so that I won't spoil much of it.
I have just seen the last episode. I was greatly moved by it, and the plot twists always made me think "Oh my god... what'll happen next?!" so I always continued watching it until the very end. The beginning of the series show some comedy relief between the episodes, but near the ending, that's where the REAL seriousness shines. Some say that the ending was just awful, others say it wasn't what they expected... I'd say that I'm in the group that wasn't expecting it to happen. It was truly shocking and made me shed a tear (won't spoil more, you'll see!~)
As the other reviewer says, it's 'probably' better to just watch it in japanese with english subtitles. Who knows, it's your decision!
Definetly worth a 5 star rating, this series entertained me like Fullmetal Alchemist and Bleach has.