I'm going to give you two reviews for the price of one here: mine, and my 11 year old son's. He loved this book - he liked the action, the characters appealed to him, and the whole idea of being a "wolf's head" (a medieval outlaw) was fascinating to him.
I was a little more critical. I thought Crispin was certainly worth reading, but I found it a little dark. I kept expecting someone to get killed horribly (and it didn't help that I already knew a little medieval history and had heard of John Ball and the Peasants' Revolt of 1381). The medieval history in Crispin was very interesting, and well-presented, and intertwined nicely into the story of Asta's son - but again, it was all a little depressing for me, starting right off the bat with Asta's death.
I couldn't help comparing Crispin with the other medieval Newbery winners I've read recently (The Midwife's Apprentice, Good Masters! Sweet Ladies!, and Adam of the Road) - and quite frankly, I enjoyed all them a lot more than Crispin: The Cross of Lead. Crispin did make a very nice boys' counterpart to The Midwife's Apprentice, I grant you, but I didn't find the ending quite as inspiring or satisfying. In fact, I didn't like the ending of Crispin at all. I felt like it left too many questions unanswered. And both Adam of the Road and Good Masters! Sweet Ladies! had a certain story-telling charm and joy that I just didn't find nestled in Crispin alongside all of the drama and the history.
However, I know my son would disagree. He hasn't read the other Newbery winners set in medieval England, and given his taste in books right now, I think it is unlikely I could convince him to read them anytime soon. I should also note that although I didn't really love Crispin: The Cross of Lead, I did get well enough hooked that I'm going to read Crispin: At the Edge of the World as soon as my son is done with it.