It was slow at the start but eventually picked up, however I found the story in The Mortal Instruments to be a much more compelling story. Granted Mortal Instruments was set in the modern world and Infernal Devices takes place some time in the 1800s, so perhaps the setting made it easier to enjoy the former.
Obviously, chronologically, Infernal Devices takes place long before Mortal Instruments but I would recommend reading the latter first. Infernal Devices does contain some origin stories and the main characters are the ancestors of the characters from Mortal Instruments, but I found this book far more interesting after having read the other series first.
I do hate to say it though, the writing seems a little weaker in this book than it did in the other series. Perhaps it was the slow start to the book.