The first half an hour is fairly breathless. Lupin goes from one chase to another. The sequence featuring a huge lorry chasing Lupin and his mate Jigen in a tiny little car is a standout moment. Then it all goes very slow and dull. It descends into chatting about clones, friends falling out and Lupin letching about. Being generous, you could call Lupin a 'ladies' man'. Being less generous, a 'pervert'. Tonally, his escapades with the improbably proportioned Fujiko don't sit well with the rest of the film. This aspect of Lupin was toned down for The Castle of Cagliostro. Good.
The villain, Mamo, is a crackingly disturbing piece of design. His initial reveal reminded me of the final reveal in Don't Look Now. I didn't have quite as strong as a reaction as I did to that scene, but he is a dirty, slinky, little, lilac-suited freak with lady curls in his long white hair. It's a shame he doesn't get up to too much villaining about. He just gets to talk a lot. And float a bit.
Still, there is something relaxing about this style of animation from the late 70s. It has a charm that CGI generally lacks. The music is pretty great too. Couldn't find the main theme from Secret of Mamo but this version should give you an idea: Lupin The Third Theme. So, if you want a film to watch when you're feeling tired, you can watch the first half hour then fall asleep. Perfect.
If you like this you could also try:
The Castle of Cagliostro, Laputa: Castle in the Sky, Nausicaa of the Valley of the Wind, Tekkonkinkreet.