It was excellent. The criticism against it (that it whitewashes the Nazi era by pretending that tons of Germans were working against Nazi atrocities) is not at all merited. First of all, because it portrays the resistance as being very small and scattered. And, secondly, because the resistance in the book is so incredibly ineffective. There are actually a number of bumbling resistance figures. There's a judge who hides a Jewish woman who promptly gets herself caught. There's a resistance cell that breaks up on account of a love triangle. There's the widow of a communist who gets bilked out of her life savings when she tries to help a man she thinks is a resistance figure. And, finally, there's the heroes of the book, who distribute these postcards across Berlin and think that the postcards are being passed from hand to hand by people who are secretly opposed to Hitler when, actually, more than 90% of them are immediately handed over to the Gestapo.
The story is a great example of the role of fiction in questions of politics. Because it begs the question: Was there any point to these activities? Should these people have just kept their heads down and tried to survive? Is it enough just to try?
And there's no good answer to that question. The book merely presents the situation. It is up to you to decide.
I will say that the book seems to come down somewhat on the side of saying that there's something valiant in the effort. But it hardly gives a ringing endorsement to its heroes.
For myself, I'm willing to go along and say that I respect the people who formed the inspiration for this book: I respect their integrity and willingness to sacrifice their lives. But if I was in their place, I probably would not have done the same. A person only has one life to live. Why throw it away for nothing?