I've been editing OSM for years. (896,339 edits in 3,427 changesets, apparently!) For me, it's all about the free data. I once got a thank you note from someone who worked for a city with a particularly large municipal park. I'd added almost all the trails to the park and other information, and they'd used it to produce a printed map for the general public. Exactly the kind of thing I'd hoped for!
Personally, I do a lot of dualsport motorcycling and most backcountry maps around here are subpar. I map tons of trails and 2track and put them on the Garmin so I know where I'm going.
OSM is also great in lots of Europe--tons of detail.
JOSM is great.
Someone just recommended Organic Maps for the phone--it's way snappier than Google Maps, but still not great with finding addresses.