Pretty underrated, but also pretty underdeveloped. I feel like a lot of the people who comment on Part 1 being the "worst part" are just people who can't get into older manga, lol. The first chapters up to the fight as the mansion burns down is one of the best, most compact stories that Araki's ever written, and could stand on its own as a great short story, but Araki wanted to do more than that.
A lot of the fight choreography was kinda meh but Araki's still developing writing combat "physically" compared to "mentally" at this point. Even when there was physical combat in his prior works (Cool Shock B.T., Baoh, Gorgeous Irene), it was a much different style compared to what came to be in JoJo's, so he has reason to be in unfamiliar territory here. Like I said before, the best stuff comes during the brawl in the Joestar Mansion.
As it becomes a quest to kill Dio and destroy the stone mask, it becomes a lot more of a standard Shonen affair, but it still stands out from others with Araki's unique style. You can really feel the terror on a page when Jack the Ripper makes his appearance in the tunnel, or when Zeppeli gets ripped in half during the fight with Bruford, and when Dio rises from the dead- Even SPEEDWAGON is afraid!
Lotsa cool characters in here, Johnathan, Dio (I think he's at his most interesting here), Speedwagon, and Zeppeli, but overall a very small cast. Only a couple other names end up being worth remembering (Straizo, Dire, Bruford, Tarkus), but all of them are certainly worth their stay. Pretty interesting concepts are thrown around, but the ones that stand out are obviously Hamon, Vampires, and the Stone Mask- And for being so early on, I have to say that the Stone Masks are such a sweet ass concept. I wish that we got to see a bit more of them in Part 3, but I can't say I'm not content with what was given to us in the first two.
I grew up with the anime (I got into it back in late middle school), but I do think it pales in comparison to its manga counterpart. A decent amount of great stuff gets left out to make an insanely brief 9 episode run out of 44 chapters. If you enjoyed the anime, read the manga, too.
A good, interesting, classic MANga that makes for a good start to the JoJo's franchise. I'd say A- or B+ is good for this one.