Basically, the whole plot is : "Okay guys, let's see who's the best street racer around here and… who's this youngster with that old car?". It's paper-thin, but it manages to deliver a very straightforward story, good characters, passion, and loads of adrenaline until the very last episode, which is six seasons (and much better visuals) later.
Despite its old age, Initial D was still a great watch, to my own surprise. There are two reasons to that, which I would like to point out:
Firstly, the direction and the rhythm more than make up for the lack of better animation and overall art. The cars could be rocks, you still would feel thrilled by each and every eurobeat-themed race. And we are talking early Playstation-level of CGI, here, which is definite proof that the shounen-style storytelling and the direction outweigh the not-that-bad looks of it. A point that remains valid for the following seasons.
Secondly, the story is very focused on its main theme—street racing; there are very few episodes that deal with what directly concerns our protagonists and their cars. Races are the most prominent part of the series and can easily last 3 or 4 episodes without you feeling robbed of its conclusion because many things did happen during the episode. Consequently, there is a lot of action, meaning a lot of races, which are supported by interesting competitors.
So, yes, it's great! And the racing genre being underrepresented in anime, the only risk you take in watching this oldie is of quickly feeling wanting more and not finding much after having binged the six seasons.
