In OD&D, one-quarter of all "magic items" consist of treasure maps. This percentage is reduced to one-tenth in AD&D, but that's still sizable. If you've ever wondered about the origin and purpose of the read languages spell, look no further than treasure maps: "the means by which directions and the like are read, particularly on treasure maps."
I've always had a fondness for treasure maps, both in the abstract and in D&D. Perhaps I read Treasure Island one too many times, but I find maps of this sort to be very "magical." They're invitations to adventure, which strikes me as very old school. Characters who find a treasure map aren't required to follow it up; there's no guarantee that it leads to anything of value to them -- but it might. Weighing the risks of pursuing the directions of a map of unknown origin is another aspect of the strategy inherent in old school play. Maps are also an opportunity for the referee to expand the world beyond his immediate campaign area and indulge in that other pillar of D&D, the wilderness adventure.
I've used maps with a lot of success in the past. Morgan Just's quest for a pair of gauntlets of ogre power included at least one treasure map that ultimately proved to be of dubious value. Rory Barbossa's map to the Isle of Dread provided a lot of fun back in the day too. I've got some maps waiting to be found in Dwimmermount and the megadungeon beneath the ruined monastery will also include a number as well. When I was a teenager, I went through a phase where I was very keen on making props for use in my campaign. Treasure maps were among my favorites to make and I went to some great lengths to make them look "authentic." I used to bury them in the mud and dig them up after the soil had hardened. Sometimes I soaked them in tea for a while and then threw them in the dryer to give them a weathered look. I also discovered that a particular kind of aerosol air freshener, when applied directly to a piece of paper gave it a spotted, almost worn-eaten look. This also gave the paper a peculiar odor. Unfortunately, it also made the map quite flammable, as I discovered to my chagrin after applying a lighter to one in another experiment to give the map a "realistic" appearance.
Judges Guild produced at least one volume of treasure maps for use with D&D. I never owned it, so I can't speak to its virtues or flaws. I seem to recall that TSR produced something similar in the 2e era, but my memory may be playing tricks on me. So, the concept of the treasure map hasn't entirely faded from the hobby's memory, even if it is a lot less prominent than it used to be. OD&D "hardwires" the notion that at least some of the loot adventurers obtain from their expeditions is deferred and requires additional effort to claim. I rather like that and plan to do my part to promote the idea more widely.