In many regions any bogus data will be spotted fairly quickly. New mappers are monitored by the community and guided to documentation if necessary, and real abuse is dealt with (either by just reverting vandalism, or blocking the user outright via the Data Working Group).

Areas with local mappers will be watched even more closely.

All in all OSM often reaches a level of detail Google Maps can only dream of. In the Netherlands the number of outright errors is really quite small, and at least everything is pretty much up-to-date. Google Maps is still showing (and routing over) a trunk road in my home town that closed over a year ago.

> A map of the world seems like sooo much work to keep updated

It's really not that bad. Major infrastructure draws the attention of people who are fascinated by highways or railway tracks. The same goes for national cycling networks and the roads and cycleways these use. Major administrative rearrangements (changes in the borders of municipalities etc.) also draw a particular group. And finally, local mappers care about their local piece of map, often down to the newest projects. As a local mapper myself I find it a joy to be the first to map a new street or add a newly assigned name to some unnamed way.

It's also a nice hobby, because you visit parts of your town to survey you wouldn't ever come otherwise.