You've probably had occasions when a friend says to you "You know the one about the rabbi and the priest?" and you say "Yes." You're both kind of let down, and often the best thing is to at least recap the joke so that you both chuckle and move on.
That let-down feeling captures this book. A lot of it seems half-hearted; McMullen points vaguely in the direction of a joke, and lets it go at that. I'd like to give him credit for subtlety, for being one of the few writers who credit their readers with any intelligence, but here, McMullen lets go too early. We see what the joke is going to be, but it never actually happens.
The previous three books in the series ended pretty well; this one doesn't tie up any loose ends, so don't buy it just for a sense of closure.
All that said... I found this book at a discount store, and I'm happy I bought it. At full price, I would not have been.