Right from the beginning, Pandora Hearts is full of twists and turns. While some of them can be guessed, what really impresses me is the number of curveballs that Ms. Mochizuki throws; the plot keeps you almost constantly in suspense, and though there are a few divergences, it never really loses focus on the central mystery of the Abyss and the Tragedy of Sablier. The nice thing, too, is that hints and reveals occur fairly regularly, so you're never wading through long pieces of filler waiting for the next plot development. The main problem with this first volume is that it takes a few chapters for things to really get started, so don't give up too quickly!
The characters are, with one notable exception, fantastic. Gil, Alice, Break, Vincent, Sharon, even Ada - everyone has hidden depths, making them much more interesting and giving them surprising psychological realism; they're not just defined by one incident or character trait. The exception is, oddly enough, Oz, the main character. I'm not sure what it is, but his development sometimes seems forced, and he's one of my least favorite characters. It's a testament to how strong the other characters are that the series is still good in spite of him.
Based on a vaguely steampunk Victorian aesthetic, the art style is gorgeous (the level of detail that goes into the costumes and settings is incredible) and really evokes a subtle sense of gothic horror. Another reviewer commented that he or she found it hard to tell the characters apart, but I think the context almost always makes it clear. It's also nice to see so many side-comics thrown in! They can really lighten the mood when the story gets to its darker places.
Finally, this series has an excellent sense of humor. You wouldn't think that you'd find time to laugh between a girl selling her soul and the main characters being forced into a hellish nightmare world, but Ms. Mochizuki has sprinkled some hilarious comedy throughout (Gil is quite attached to his hat, as you will see). It really helps to keep things in balance, and it sets Pandora Hearts apart from other thrillers.
ADVISORIES: I wouldn't recommend this to anyone below the age of 13. There's not much in the way of foul language or sexually suggestive material, but there is quite a lot of violence right from the start. If you can't handle blood, you might want to give this one a miss. It's actually quite similar to D.Gray-man in general feel (except that the plot has yet to dissolve into an incoherent mess).
Combining shades of Alice in Wonderland, Edgar Allan Poe, and Victorian England, Pandora Hearts is an incredible thriller with large doses of the macabre and screwball humor. Even if you're not generally a manga fan, I'd recommend it to anyone who enjoys a good, supernatural mystery with strong characterization. It's one of the most engaging manga series I've ever read, and may be my favorite after Black Cat!