Lupin III is a classic, in the same way that the spaceship in 2001 or the first chord of 'Hard Day's Night' are classic. Or in the words of Van Morrison. "It aint' why - it just is." (though I'm not even sure what Van meant by 'it')
I remember about two minutes into the first episode I watched of Lupin, thinking "ah, so this is where they got Cowboy Bebop." The similarity is excuriatingly hard to miss, and I might say that CB lacks Lupin's natural humour, and effortless style.
Those who have seen 'The Castle of Calgiostro' will be familiar with the characters, though in the TV series they make quite a lot more sense and seem less thrown together. I love Jigen's attitude to his boss, Lupin, which is somehow obedient without being cowed or subservient. Fujiko's unattainable, star-crossed relationship with Lupin makes more sense in the series too.
Lupin is the world's master theif, bringing a distinctive sort of 007'ish anti-hero feel to the screen, especially in the early episodes, which were virtually produced in the 60's. He also has a vulnerable side, whether it be for pretty girls, Fujiko, or some unexpected sentimental distraction. Despite his genius as a thief he is prone to emotional disappointment, but shrugs this off with a panache which exudes class.
And that class is hardly surprising. Not only were the early episodes mostly directed by Miyazaki, but other Ghibli luminaries involved included the director of 'Grave of the Fireflies', and key animator in the 1971 series was Yoshifumi Kondo - who 24 years later Miyazaki allowed to direct maybe Ghibli's best movie, the superb 'Whisper of the Heart' (he died not long afterwards, and Miyazaki has expressed regret that he didn't give him his own movie earlier).
If you liked Cowboy Bebop, you have to catch Lupin. It's even better. If you just want to catch early classic anime, the only rival I can really think of for Lupin is Kimba (which is of course utterly different, but still...)
A word of warning: I'm not sure which seasons of Lupin evailable are bootlegs and which are legal. And I don't know how you would tell anyway. The three or four boxsets I have are all subtitled Japanese.
Lupin III is a classic creation, a classic series. Not my absolute favourite animated series ever, but easily deserving of its four stars.
One word of caution: this review is based on series 1 and 2, but especially 1. I haven't seen 3.