Death is always wonderful in the Discworld books, even during the briefest of cameos, and so it's no surprise that they're the absolute highlight of this book. 

However, given the nature of Death the character they're often explored through other people in the books and so when it comes to the Death series (such as it is) the supporting characters and their storylines are of the utmost importance and unfortunately I've always found Mort to be quite bland as character which is a bit of a let down given the fascinating circumstance he's been placed in - namely that of what happens to a person burdened with the charge of taking on the duties of Death. 

There's also the fact he spends the majority of the book risking damaging reality because he saw a pretty girl for five minutes, but then again he is a teenager so perhaps that part is accurate enough if not particularly enjoyable.