When you watch the trailers and then the first episode, Princess Principal looks a lot like "Lupin with girls". There's Ange, the leader of the action and a master of disguise like Lupin; Dorothy, the driver and best friend like Jigen, and also an irresistible femme fatale like Fujiko; and Chise, who is clearly Goemon's daughter. There are car chases, building infiltrations and clever plans to complete the mission. In one early episode there's even a jump through a giant clockwork (hello, Miyazaki! hello, Cagliostro!), and of course there is always a princess to be saved (hello, Cagliostro again!).
But then, quickly the plot develops and you start to discover that there is much, much more. Things start to get dark, and often heartbreaking. Bad people die, good people die, often right in your face when you were rooting for their survival. At the same time, the plot is exceptionally well written, and will continuously surprise you with twists that almost never look forced or unbelievable. In fact, every detail will always go in its right place in the end. The story will also bring in more and more themes: what is a lie and what forms your identity are the most obvious, but there will be implicit, surprisingly fresh commentaries on loyalty, war, poverty, social inequities (the social depiction of late 19th century London, full of poor people and orphan children, is quite precise), working class slavery, fatherhood, broken families, and even racism and early relationships between Japan and the West. Except for a couple of filler episodes (more relaxed, but never funny; don't expect any real comedy) each episode will leave you emotionally exhausted, but also waiting for the next one.
The series is also technically brilliant. There are a few directorial choices that you won't forget, like the ending of episode 6. The musical score is superb, as any Yuki Kajiura score is; Lupin-style jazz plus classic arrangements plus the right bit of tension at the right point in time. The animation is also often notable, especially in action scenes.
The many prizes that this series won are well deserved, and it is incredible that it was not a big hit; I suppose that the main reason is its darkness and emotional toll, or perhaps people were fooled by the appearance of "pretty girls in a high school" (by the way, this is one of the few cases where the prettiness of the girls is credible, and never gets in the way of the story; fanservice is almost non-existent). Let's hope we get more of this in the future (can't wait for the movie). Unless you are only interested in anime because of comedy and big boobs, you should absolutely watch this series.
