Before I saw this show, I was completely against anime. That's because most of the stuff that's popular is the wide-eyed mindless "I fell on you and accidentally touched your boob hehehe" or "Haha the stupid chick found porn under his bed" drama type stuff. However, this was completely different.
The greatest thing about Lupin is you get so much Lupin. You get this series, the first and third series if you can get ahold of them, a lot of movies (Though I only like a few), 2 manga series (One hasn't completely been out yet though, and there are others that haven't been released in English) as well as Lupin the first stuff (Novels from Maurice LeBlanc that Lupin III is based on, as well as movies based off those books, most being hard to get ahold of though), some people don't make their Lupin stuff very Lupin-ish though, so even if you don't really like what Lupin is, there still is probably something with the Lupin name you'll like.
Lupin III is about the greatest thief in the world and his two friends and partners, as well as a woman who he's gaga for that milks him for all he's worth. However, he also maintains a heart (and no, this isn't one of those drama-ey shows either) and does end up saving lives.
His main partner is Daisuke Jigen. He's the gunman. He never misses, and nobody drawns faster then him. He has a strong dislike for women, most likely because the women he falls in love with die or betray him.. Or both.
Then there's Goemon Ishikawa The Thirteenth. He's based off a real person of the same name. His sword can cut ANYTHING (except devil's tongue jelly, as I've heard), literally. The second he pushes up his sword, it's already over. He often complains on having to waste his sword cutting useless things.
Then there's Fujiko Mine, who's said to be based off various models from Playboy. She uses her sex appeal to get anything she wants from men, often betraying Lupin in the process. Of course, Lupin always forgives her, because he's crazy for her, but Jigen and Goemon knows she's more trouble then she's worth.
Lastly, Koichi Zenigata. Based off Heiji Zenigata, a fictional officer that defeated people just by flinging coins. Although, Zenigata isn't quite as skilled as him. Well, not with Lupin anyway. He's sharp, but often is the comedy relief character when it comes to Lupin. His whole life is chasing Lupin, and if Lupin died, his life would be worthless in his opinion.
However, Lupin's not your average thief. He's a gentleman thief. He always leaves notes saying when and what he's going to steal. He does this all with the understanding that he doesn't kill anyone, and the cop who's always after him doesn't kill him either. Lupin is so huge, that many, many, many things have also been made after him, and don't be surprised if you find many references to him in other shows, movies, etc.