It's easy to see why this launched the career of Dietrich, and it has nothing to do with her acting. She steals the show from under Jannings' nose in a strange echo of the plot of the film. But this is unfair to him, as he's clearly the best thing in it and it's easy to see why he was unamused to be upstaged by a pair of gams with an attitude. But hey ho, this was 1930 and Dietrich was just what everyone wanted. The right woman at the right time.
Anyway, to the film. Like many others from this era it's best described post-silent, because the dialogue is sparse and everything is about the expressions. Von Sternberg is not quite Fritz Lang of course, but his direction is interesting, and the design suitably evocative. The story is an old classic, and well told, but the shift in tone is a little jarring and unbelieveable. However , I'm aware it's more of an allegory, so we can let that slide. Overall, a very interesting piece that stands out well despite being old ground (even then!), and you can't help but feel that were a similar film released today, all the accolades would go to Emil Jannings' character rather than Dietrich's, or at least hope so!