Reading Guernica took me three weeks--not because I've been extremely busy, but because I had to motivate myself to pick it up and keep going every time I put it down.
I know almost nothing about the history of Spain (except for a little from this book and a little from reading The Last Queen), and Boling didn't really help me to understand the politics at work, so I had no idea whose motives were what. Boling may have had pure motives for that; he says in his notes that he "tried not to tax the reader with elaborations on the complex and volatile politics at work at the time," but I would have been grateful for some help in understanding the bigger picture.
Though he creates some interesting characters (many based on historical figures), those characters are too perfect--the graceful, beautiful women who make perfect wives; the strong, considerate men who make perfect husbands; the charming children who become one of the aforementioned. Even characters who started off with alleged weaknesses (Miguel couldn't talk to women) turned out to not really have those problems they were assigned. Not only that, but there were too many characters to keep track of; I couldn't keep them straight, which gets frustrating.
Also frustrating: the book was predictable. I'm not a person who tries to guess the plot ahead of time, but with this book what's going to happen is so obvious. And I'm not just talking about the destruction of the town, but of pretty much the whole plot.
I know some people who have read this really liked the inclusion of Picasso amongst the characters, but I only found him, at first anyway, annoying--another character to keep track of. In fact, I thought the whole first part of the book could have been left off, the pertinent details included elsewhere. I was, however, fascinated by the description of the painting of Guernica, which Picasso painted after the German attack on the town. I'd never seen it, so I had to look it up.
Overall, the book was mediocre, but the research was impeccable. History buffs will likely enjoy it far more than I did. If you are not a history buff and want to give it a go, I'd highly recommend that you at least read up a little on the Spanish Civil War--on Wikipedia or some other (more reliable) reference site--beforehand.