You don't watch it for the plot, the acting or frankly Harpo's harp-playing (good as he is), you fast-forward through those bits to get to Groucho's one-liners and the Chico/Harpo slapstick stuff. The lines are extraordinary:
Chico [playing same piano piece over and over]: I can't think of the finish!
Groucho: That's strange, I can't think of anything else.
Groucho: One morning I shot an elephant in my pajamas. How he got in my pajamas, I don't know.
Groucho: You're the most beautiful woman I've ever seen, which doesn't say much for you.
Groucho [following convoluted conversation which somehow ends up with them discussing an imaginary property map on a table]: I don't like Junior crossing the tracks on his way to the reform school. I don't like Junior at all, as a matter of fact.
Actually, don't read these quotes. Get the film and watch it yourself. Then imagine being in an audience watching the stage version of it, and heckling Groucho just to see what he would come back with, which people used to actually do - a far better way to edit a script than paying someone to do it for you. As they moved on to Duck Soup, Horse Feathers and the rest it all became a little more carefully produced and lost some of its edge, although the overall films are probably of a higher quality, at least in terms of professional movie output.
This is raw stuff though, straight off the stage, and that's what makes it so good.