It's a fantastic character study.
This is a work that may seem outdated from an outsiders perspective. Let’s be honest here it's a battle shounen made in 1968. It's not necessarily spring chicken by any means. But Joe offers far more than what meets the eye despite all these seemingly understandable fears that other works that came after it exceed its value.
I'm here to say this. Nothing before or after Joe could ever replicate what it did within the context of a shounen. Joe is a classic in the truest sense. Its influence on other works is very evident but its style is completely unique to its own.
Unconventional MC
Joe is the farthest thing from a prototypical shounen MC in terms of how his arc is structured and how he is characterized. Throughout at least one third of the manga he is portrayed as arrogant, rude, impulsive and downright sinister. Obviously, there's a context to why he's like this, he did not have the best upbringing being orphaned and poor. But that's more subtext than something explicitly told to you.
You have to be able to not necessarily forgive all of Joe's actions but at the very least empathize with him and desire for him to improve. This is where the strength of Joe's arc comes from. He primarily shows development through action. Joe isn't open to sharing his feelings on things and because of this every time he does something that isn't typical of what we usually see, it's very noticeable.
You see this distant and troubled person improve gradually throughout the years slowly, coming a long way from where he initially was at. Due to how immature Joe's character is he becomes extremely susceptible to change.
This choice to make Joe unlikable initially only served to further strengthen Joe's character and I wish more shounens would make an effort to adopt this style of characterization. Because I feel this slow burn makes Joe's character very special with how negatively he is portrayed in the beginning of the story. It promotes a desire from the reader for change within him. Something I do not get with other battle shounens where the MC is usually portrayed in a more positive light throughout. Joe is true to himself for better or worse. That's the strength of his character.

Show Don't Tell
No this is not merely an element of Joe's story. This is a core part of it. Without it Joe wouldn't be nearly as effective as a work of fiction.
When I think of Joe, I think of actions speaking louder than words. Joe to me is a story composed of stares. It is a boxing drama first and foremost, but the story it tells with the stares is far more effective than any eloquent speech a character could ever tell me or any punch they could throw. A cold stare speaks volumes in this series and its used throughout. Piercing glares that could see through any facade.
With so many shounens giving exposition dump after exposition dump, the use of visuals to tell a story is a huge asset for Joe to have. Especially with how it's implemented within its story, it's not something that you merely forget and say...

Wow how neat

Joe constantly challenges the reader with the scenarios it throws at you. Sometimes it may seem that a character is slightly underdeveloped, or an arc didn't reach its full conclusion, but that's purposeful. Joe is concise and earnest. It does not want to waste your time with meaningless details. Instead it shows you how a character is impacted by an action and lets you decide if it was enough. The confidence of the mangaka to write a story with these type of visual motifs (which I've dedicated to discussing the stares primarily, there is more subtext to unravel but I rather let you see for yourself) speaks volumes of his skill. If you want to see something that doesn't spoon-feed you all the answers without necessarily leaving you questioning everything. Please read this work. It's storytelling 101.

Fantastic Structure
With many shounens nowadays seemingly ending on wet farts or stagnating due to fear of experimentation or backlash there exists works like Ashita no Joe. One of the best things about Joe is that it's a complete vision and because of this is fully finished. Yes, I get what I said was redundant, but that's for emphasis of this point.

You will not read many series that go past the 100+ chapter mark that retains the quality of when it initially started and peaks at the end. Joe once again breaks traditions and in my opinion peaks at its final chapter. The author had a long term goal for what this story was initially set out to do and whether or not if Joe dying was a stroke of genius or a happy mistake is neither here nor there (final chapter spoiler).

Now I may sound like I'm providing fellatio to the ending of the series moreso than the structure of this series. But the ending is only effective because of the authors long term storytelling. It takes two to tango. Due to him building to the climax of the series gradually throughout several arcs in Joe's life. The story is thematically cohesive and tells so much with minimal tools. It's an exercise of fantastic writing when the climax is actually the climax. Oh, dear I did it again...

Joe doesn't offer a great structure from just an ending standpoint (even if it is the best example of it), it also offers a fantastic structure in general. The manga rarely if ever actually wastes your time. Every arc is relevant in some way to Joe's growth as a person and because of this story being mostly a character study, that's all you need. Of course, there are some fights with higher stakes than others, but each one is integral to Joe in some form or another. He learns from his opponents and never comes out the same from all the main fights. The mark of a truly fantastic structure is if all parts contribute to growth for the characters or narrative and Joe provides both and more with its arcs.

Conclusion
Read Joe. It's great and worth your time. It's highly influential and aged magnificently. It's a work with a lot of aspects to discuss and despite it never reaching the west officially is still very much loved in Japan. If you love character dramas, there are very few that I've read that are better than this. Even fewer sport manga's that read so far ahead of its time like this one.

Negative Thoughts
Perfection doesn't exist.
A masterpiece to me is fantastic IN SPITE OF THAT!
So yes. Joe has some flaws.
It's rather slow in the beginning despite being very unique initially. This is due to the character growth of Joe taking longer compared to the average shounen MC along with the lack of boxing within a professional setting for a while. This slow build to me wasn't an issue at all due to me being a patient reader, but I can see people being frustrated with this slower form of storytelling.
The story is very grounded in tone and structure for the most part. But there is a fight in volume 19 that leaves a lot to be desired. Going from this consistent stream of realistic boxers with very real motivations to a joke fight. Despite this, think the growth of Joe from this fight was strong and the aftermath of this fight had a massive impact on various characters (which might not have been gained if it didn't happen). Though, I'm willing to admit that it could have been handled slightly better.
Fight sequences are outdated. But to me this has a far bigger appeal to drama fans. Which is why I praise it so highly. If I wanted great action sequences, I would read something like Ippo, Naruto, Jojo, or Hunter X Hunter. What these fights get accomplished is that they all have massive stakes associated with them. Not the actual fight choreography itself (which still isn't bad though very simple).
If you don't like Joe. There is zero chance you will like this manga. If you don't want to see him grow and just hate him from the get-go while not being able to accept that he will grow. Please don't read more. You will only disappoint yourself.

Extra Thoughts
Joe anime is far less subtle than the source and overstays its welcome with some lackluster filler arcs and breaking the rules of "show don't tell" which Joe perfected. The benefit to it is that the animation and directing is breathtaking. Along with strong voice performances and music, otherwise I wouldn't recommend it at all over the source material. I am of course talking about the 2nd season of Joe. The first season is much slower paced than the source material. Also it's very dated with some really bad audio mixing and lazy animation throughout. It does have its moments though, which I would honestly just watch through internet videos rather than watching the whole show. The second season is fantastic (I personally gave it a 10, but that's WITH context of the source material). Though the criticisms I gave still apply to it.

Ippo has a lot of neat references to Ashita no Joe. Sendo being the most obvious example (being a homage to Joe Yabuki). If you want to read a completely different take on boxing that emphasizes the sport aspect, I would recommend Ippo. Even if it's very inconsistent.

Joe is a very important work to me. It made me realize that a story can be amazing and ageless no matter what time period it's from. It even holds the distinction of being the only manga that made me cry. A feat very few works of fiction that aren't visual novels have done to me. It's a work of passion.

Hey hey hey blues.
Nothing but rollin' the blues.
The attitude of this guy is unforgettable.