I'm a little over 1/4th of the way into Part 5, now. Araki's art improves so much that it's ridiculous. His stuff for Steel Ball Run and JoJolion is some of the best artwork I've seen.
Parts 3 and 4 couldn't be more thematically different - Part 3 was a grand adventure and you know who the villain is right off the bat, and Part 4 all took place in the same town and was all about figuring out who the real villain is. I got big Persona 4 vibes from Part 4, actually. It even had its own Brosuke character.
Part 5 is interesting, and I love the character design in this part. A boy band mafia? Sure, why not! The stands keep getting progressively more complicated and creative, too. Giorno's hair is a bit annoying, though. He looked much better with normal black hair. I really like Araki's idea of making Dio's son both a JoJo and a protagonist.