TL;DR: Though many of its episodes are more resembling of an old saturday morning cartoon, the quality in writing and funny (although quite campy) jokes make this series a superb comfort watch when you just want some well-done baseline Lupin III. Perfect for someone new to the Lupin III franchise to begin with.
Now to get into the long-form review:
This. This is what Lupin III should be in all its glory. This ancient anime set the standard for a dynasty that is still going to this day and is gaining more and more praise and fame across the world. Even though it is only the second series in the Lupin III franchise, it has managed to surpass the first by adopting a much lighter tone (although the later episodes of part 1 also laid the groundwork for this tonal shift), Yuji Ohno's superb work on the soundtrack and most crucially; the first English dub from what can now be called the greatest dub cast of the franchise.
It is for that reason why I recommend you watch the 2004 Geneon dub: even though its writing is heavily modified from the original Japanese dub and its animation is a bit shoddily adapted to this fact (especially in the first couple of episodes), the sheer hilarity campiness of the early 2000's dialogue can't be found anywhere else in the franchise.
I wouldn't really advise to watch this series from episode one 1 to 79 (yes, unfortunately only 79 of 155 episodes were dubbed), rather looking up a list of its episodes online (for instance here in IMDb) and just picking what you find interesting from reading the synopsis; due to a complete lack of continuity you can just tune in with any episode and get a unique story.
The episodes themselves can be categorized into three categories: 20% are true classics, 65% are fun for a comfort watch and 15% range from mediocre to downright awful. It is for this reason that I implore any new viewer to just explore the series on their own pace to test the waters and see what they like.
The series does have one pretty annoying aspect that can turn some viewers off of it; the animation. Even though it isn't bad for 1977, it still leaves a lot of room for improvement when you are used to the modern standard of animation in anime (and the latest installments of Lupin III). Some backgrounds loop too early, characters stand still for a bit too long, et cetera et cetera. Nothing out of the ordinary for cash-strapped TMS. Especially the English dub also suffers from poorly looped speech animations in its earlier episodes; if you have a strong distaste for poor animation, this series might not be for you.
However, the best part for the keen listener will be the soundtrack. This was the first series in which Yuji Ohno was involved with the soundtrack, and he hasn't been absent ever since for a very good reason. Comparing the sound design from part 1 to part 2 is comparing the sword to the rifle; there is simply no competition. In the series Ohno's masterful jazz fusion tracks are played near-constantly, and if you appreciate "70's groove" and a bit of spy movie aesthetic this series is practically Valhalla.
In conclusion; this is the true establishing ground for what Lupin III became. The English cast, the music and the style all originated from this series, and it still holds up as the baseline for any experience watching something from this storied franchise. Lupin III has his classic look, his classic gang and his classic adventures. If you want to get into the franchise without any previous engagement with Lupin III, here's where you start.
Now to get into the long-form review:
This. This is what Lupin III should be in all its glory. This ancient anime set the standard for a dynasty that is still going to this day and is gaining more and more praise and fame across the world. Even though it is only the second series in the Lupin III franchise, it has managed to surpass the first by adopting a much lighter tone (although the later episodes of part 1 also laid the groundwork for this tonal shift), Yuji Ohno's superb work on the soundtrack and most crucially; the first English dub from what can now be called the greatest dub cast of the franchise.
It is for that reason why I recommend you watch the 2004 Geneon dub: even though its writing is heavily modified from the original Japanese dub and its animation is a bit shoddily adapted to this fact (especially in the first couple of episodes), the sheer hilarity campiness of the early 2000's dialogue can't be found anywhere else in the franchise.
I wouldn't really advise to watch this series from episode one 1 to 79 (yes, unfortunately only 79 of 155 episodes were dubbed), rather looking up a list of its episodes online (for instance here in IMDb) and just picking what you find interesting from reading the synopsis; due to a complete lack of continuity you can just tune in with any episode and get a unique story.
The episodes themselves can be categorized into three categories: 20% are true classics, 65% are fun for a comfort watch and 15% range from mediocre to downright awful. It is for this reason that I implore any new viewer to just explore the series on their own pace to test the waters and see what they like.
The series does have one pretty annoying aspect that can turn some viewers off of it; the animation. Even though it isn't bad for 1977, it still leaves a lot of room for improvement when you are used to the modern standard of animation in anime (and the latest installments of Lupin III). Some backgrounds loop too early, characters stand still for a bit too long, et cetera et cetera. Nothing out of the ordinary for cash-strapped TMS. Especially the English dub also suffers from poorly looped speech animations in its earlier episodes; if you have a strong distaste for poor animation, this series might not be for you.
However, the best part for the keen listener will be the soundtrack. This was the first series in which Yuji Ohno was involved with the soundtrack, and he hasn't been absent ever since for a very good reason. Comparing the sound design from part 1 to part 2 is comparing the sword to the rifle; there is simply no competition. In the series Ohno's masterful jazz fusion tracks are played near-constantly, and if you appreciate "70's groove" and a bit of spy movie aesthetic this series is practically Valhalla.
In conclusion; this is the true establishing ground for what Lupin III became. The English cast, the music and the style all originated from this series, and it still holds up as the baseline for any experience watching something from this storied franchise. Lupin III has his classic look, his classic gang and his classic adventures. If you want to get into the franchise without any previous engagement with Lupin III, here's where you start.