It's quite pretty and the first two thirds are great (in premise and execution) but after that it kind of falls apart by adding too many over-complicated stands, dragging the ending on for quite a bit and pulling story elements out of its ass, and then screwing up with the defeat of the main villain. It's got a kind of very shitty ending.

To reiterate, the first two thirds are great:
The premise: Mafia Jojo, having to escort a person across Italy and defend them from assassins along the way.
The art: pretty goddamn great, with memorable characters, though the designs keep getting more and more ridiculous (and distracting) as the story goes on.

But towards the end all of it kind of deteriorates, and the ending is pretty goddamn weak and left a pretty bad taste in my mouth.
Do yourself a favor and stop following Jojo after finishing part 4.

It's an enjoying read with some really interesting stuff, but my goodness can I not get over that ending. The only reason I don't give it a lower score with regards to story is that the first two thirds are really good, but the last third brings down the score significantly.