I feel like I've earned a thumbs up alone for watching this 4 1/2 hour silent film.
Normally, films of such length are epics with huge sets, big battles and vast landscape shots.
Dr. Mabuse is essentially a character study.
This is by far the most elaborate silent film I've seen so far - with many many intertitles to develop the characters and the plot. And it definitely needs its time to fully treat its subject.
Klein-Rogge stands out as the eponymous villain, his masks and disguises are as impressive as his acting. He is amazing and he needs to be if you take into account how much the film depends on his character.
Lang was again ahead of his time (like in every genre he has worked) - it's groundbreaking and influenced psychological thrillers and character studies for years to come.
Mabuse is a notorious gambler. He plays with human beings and fates. His addiction is most prominently depicted in two scenes with the two ladies (Cara Carozza and Gräfin Told) of the movie who both point out that his gambling will create repercussions someday.
Also impressive is the way Lang lets his protagonist get away with his crimes over and over again - he's the bad guy but for about 4 hours it looks like he'll succeed. Evil has the upper hand for a good part of the film which is remarkable for a film of this period.
Mabuse counterfeits, creates havoc at the stock market and consequently ruins the economy and his ability to control the minds of others is Lang's response to the main problems and fears of post-war Germany.
Of course it's long and you need some dedication to endure it but it's also quite sophisticated (especially for a silent) and it needs its time to develop and there are merely any meaningless scenes.
Fritz Lang yet again shows that he's a master of dramaturgy, symbols and storytelling.