This novel is hated by many generations of Italians.
Poor Alessandro Manzoni! He's not to blame for the bad reputation of his book. The moronic Italian educational system is the only guilt of the assassination of I Promessi Sposi in most of its readers.
Just consider this: in Italy we study this novel at primary school, at secondary school and at high school. Every single time restarting from its beginning. Therefore it's pretty obvious that I Promessi Sposi becomes one of the worst nightmares of every young Italian pupil. I don't know why things don't change. Teachers and professors are simply obliged to include this novel in their programmes just like their students are forced to learn whole chapters of the book, being questioned on them.
Nevertheless I Promessi Sposi is a very good, intense and some times cinematic fresco of its own times, giving the best and most vivid description ever of the Great Plague in Milan during the 17th century.
If you haven't been an Italian student I think you have to get a chance to this novel together with "Le confessioni di un italiano" by Ippolito Nievo which is even better.