As you can imagine, in the early days, it wasn't much to look at. There were networks of roads, though most of them weren't named, and a lot were missing. You could sort of make out where you were, if you knew the place well. The interface was also somewhat slow and clunky, compared to Google Maps (a variation on whose draggable-tiled-map theme it was).
Some are impressively comprehensive; for example, OSM's maps of Reykjavík (and, indeed, Iceland's second city, Akureyri) and Buenos Aires are as thorough as anything you'd expect from Google, were they to bother. Harare, whilst looking quite sparse, is more detailed than on Google's maps, and the Papua New Guinean capital of Port Moresby seems fairly comprehensively drawn. Even Pyongyang has a surprising amount of detail (though one probably can't blame OpenStreetMap for most of the streets being seemingly unnamed); I imagine that as soon as North Korea allows unrestricted tourism, long before the first McDonalds goes up, tourists will be dragging cached OpenStreetMap tiles on their jailbroken iPhones as they negotiate its broad Stalinist boulevards.
Being based on free data, OpenStreetMap has other advantages over its commercial cousins. Each map comes with an Export tab, which lets you grab the displayed area in a variety of formats, from rendered pixmaps to SVG or PostScript to the actual raw data. With it being under the Creative Commons, you are free to do as you like with the data (subject to a "share alike" proviso if you commercialise it). And with it having the agility of the Wiki age, OpenStreetMap is starting to steal a march on its competitors; for example, it was the first map to show Heathrow Terminal 5 correctly.
Of course, OpenStreetMap is by no means perfect. parts of the world are still uncovered (much of the Russian interior), or only covered with major roads (much of Africa). And their rendering algorithm doesn't seem to do Chinese or Japanese characters, rendering most of China's place names as rows of boxes. (If one is picky, one could request transliterations of foreign character sets; perhaps this could be done as user-selectable layers.) There is no route-finding capability (of the sort that Google Maps has). But all in all, OpenStreetMap is very impressive, and a spectacular success.
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