It's fun to see one of the first sci-fi epics. Some of the underwater scenes are pretty cool (if you keep in mind that they were filmed a century ago).
But on the whole this hasn't aged well. My main complaint about early films in general is that they so rarely trust the audience. Everything must be shown and explained. This film feels like it includes every inch of film that got shot.
As an early example of adapting an SF masterpiece, 20000 Leagues sets a precedent of mostly ignoring the source material and inserting the novel's characters into a much weaker story. I mean, in this version, Nemo prays for vengeance over his wife's corpse, and apparently decides that the best way to achieve this is to build a submarine and skulk around for 20 years.
Of historical interest only.