My review would be 3.25 stars on average, but sadly, that's not possible.
The thesis is in many respects highly questionable. Dundes posits first that there is a quality known as national character. Second, he posits that the German national character is highly scatological, which he bases on what another reviewer has more aptly termed an analysis of German colloquial speech and writing. (A folklorist may think of colloquial speech and writing as part of folklore, but the layman probably thinks more specifically of oral storytelling when he hears the word "folklore".) This second argument may be the best-supported in the book, though Dundes doesn't go far enough in comparative anthropology to demonstrate that this quality is unusually German. The most questionable part of the thesis is in his conclusions regarding what light this supposed anal-erotic character of the Germans and their language is supposed to shed on German history. If you accept the Freudian thesis that anal-eroticism is the underlying cause of parsimoniousness, orderliness, etc., then this part of the thesis makes sense on its own terms. But what if anal-eroticism were merely a parallel aspect of German "national character" (accepting for the moment the notion that there is such a thing), along with parsimoniousness, orderliness, etc.? And there are other arguments to explain, for instance, the rise of Nazism--historical, philosophical, etc.--that do not require this Freudian analysis, which Dundes does not consider. The thesis may be fine for you if you are already a Freudian, but I am not so easily convinced.
The content, on the other hand, is wide-ranging, well laid-out and entertaining. Ignoring the thesis, this makes excellent bathroom reading, actually. And who but the most squeamish of prudes doesn't like a poop joke? (If anything, this undermines Dundes' thesis more--we all find this stuff funny. Dundes admits that, but doesn't treat it as seriously as he should, which is somewhat disingenuous.)
A last note: I was walking with a friend in Munich, when I happened to repeat to him a favorite expression from the book--"Wie der Fisch stets in dem Wasser lebt, so die Scheiße in dem Arschloch klebt." An old woman passing by gave a hearty guffaw. She was perhaps seventysomething. So the book could be very useful to anyone who wants simply a handle on (a certain class of) German colloquial expressions. The book gave me a small moment of humanity, in the form of a turd.