If you like your science fiction to make at least a pretense of scientific sense, Terry Pratchett is not your guy. Even though he references Stephen Hawking and his ideas, the time travel here is more akin to magic, except that even magic follows some rules. There is no attempt at explaining why or how it works, or even having it work in a consistent way -- Pratchett is more interested in using it to propel his riffs on whatever amuses him.
The first half of the book can get a bit tedious, as Johnny and his friends mope and bicker, and Johnny sometimes seems like a candidate for Prozac. Once they have a clear goal, though, the story picks up considerably, and becomes exciting and suspenseful. But Pratchett's fans, while they want a good story, mainly come for his clever British humor, and there's plenty of it here, though some seems a bit more strained than usual. In the Pratchett canon, the Johnny Maxwell Trilogy is easier to read and deliberately provides more to talk about than his other books and series. So, while it's generally weaker storytelling than, say, his Bromeliad or the Tifanny Aching Adventures, it's a good choice for discussion groups.