Depends on the application. The developers that made Valhalla (Mapzen) recently hired my doctoral program office mate Indy Hurt to answer some aspects of that question. She's on Twitter at @indymapper. Elsewhere in academia, Muki Haklay has been studying the problem -- here's a link to one of his original papers that can provide a seed to find his more recent work: http://www.ucl.ac.uk/~ucfamha/OSM%20data%20analysis%20070808...
There are also numerous dev gatherings that examine OSM applications and data quality. One starts in a few days: http://stateofthemap.us/program/ You might check out the program and see if your application area is addressed.
Summary: OSM data offers the best free geospatial vector data available. It is a good starting point. For serious commercial applications, however, the data are rarely ready "out of the box". You will almost certainly need to improve/transform/annotate the data for your own purposes. Keep in mind that consequential improvements are expected to be submitted back to the OSM project.