While the story feels a little overlong and maybe gets too absurd towards the very end, it's a really fun film. It's certainly more coherent than the later Gold of Babylon film, and while it's not as much of a general audience crowd-pleaser as Miyazaki's Castle of Cagliostro that followed it the next year, it's much more faithful to the original Lupin III comics in terms of humour, art and characters. Their relationships are explored in a really raw, mature way, more so than any of the other Lupin films. The sexuality is more overt (though still not nearly as explicit as the new 2012 series) and long-standing tensions that are usually only briefly mentioned finally come to the surface and affect the storyline, such as Jigen and Goemon's differing philosophies, their mutual resentment of Fujiko, and Jigen's brother-like relationship with Lupin. Similar to what I said in my post on One Piece Movie 6, this stuff probably won't be as enjoyable for viewers who aren't already familiar with these characters, but I think it would still be entertaining.
It's probably the funniest Lupin movie, which is good because Lupin is supposed to be a comedy, something that the new series doesn't seem to realize.
It's also a very atmospheric movie. The desert scenes feel almost unbearably hot, because of the extreme lighting and the music.
I think Cagliostro is probably a better movie overall, even if Lupin's personality is severely watered-down in it. It simply has better, more focused storytelling, but obviously it's hard to compete with Miyazaki in that department. On the other hand, Mamo is a much more authentic, pure Lupin experience, so if you're looking at it in terms of how it fits into the larger franchise, it might win out. Their contrasting approaches are exemplified in their animation. Both are well-animated, but in drastically different styles.
Cagliostro (seen above) has soft, rounded, "cute" designs, and the movement is very naturalistic and smooth. The characters are basically stock Miyazaki faces and designs dressed up as Lupin and co. Mamo (as seen in every other picture) has lanky, angular designs with exaggerated features, and the movement is more stylized and over-the-top. I can see why this harsher style wouldn't appeal to as many people, since the characters sometimes resemble stick figures, but I think it works just as well... Apples and oranges, you know...
Anyway, this post was basically just an excuse to share some cool drawings from the movie, so here are some more.
This sequence, where they look inside Lupin's mind, never fails to make me laugh. We see live-action photos of naked women, with Zenigata and Fujiko popping up and finally Lupin himself, seemingly in a commercial for some orange candy.