I have read all the Discworld novels and most of them I rated 5 stars for enjoyment. This book, however, would get 7 stars for enjoyment, style and the ideas behind it. My friends usually borrow my TP books but so far they have not been able to put their hands on this one!
There really isn't much of a plot (depending on your point of view of course), but the action is continuous and the punchline - although a little bit expectable - quite striking.
I got to meet one of my favourite characters in a bit more detail - Susan. We meet a new guy - Lobsang - whose namesake actually has some achievement in real-life buddhism. Also all the other nicking from 'well-known' ideas, films, etc will keep your head spinning until the last page, because you start wondering after a while if you have missed anything.
Another thing I love about Terry Pratchett is his thought-provoking style. Even a unenthusiast of physics like me has been inspired to read up a little more on the subjects of time and relativity thanks to this book.
I agree with the others that this book is not for the beginning Discworld novel reader, since most of the characters are already well known from the other books and their personalities - and in some cases personifications - are well developed there up to this book.
Once you get to sit down with this book you only have to do two things: read and enjoy!