One of the best pop-culture foils/villains of all time in terms of high-concepts. The grim, iconographic superhero at all times concerned with the sanctity of life gets a colorful, inviting-looking arch-villain who doesn't value it at all. A hero all about his origin gets a villain who doesn't have one. I'm not sure it was even intentional at the time of the Joker's creation, but he and Batman complement each other so well it's kind of genius in its simplicity.
That said, I'm not always as much a fan of him as I am in execution as I am in concept. I think it has less to do with oversaturation and much, much more to do with being overwritten when he's there. Death of the Family is just absurd; he murders whole police stations and apartment complexes in minutes. The concept of saving him for stories couched in shocking one-upsmanship is something I think has really given me a distaste for his appearances. Frankly, I'd rather he show up more often if it means he can just be a constant, lower-stakes thorn in the side again; just be present for the mirroring qualities he provides.