A very well documented article "The Street Network Evolution of Crowdsourced Maps: OpenStreetMap in Germany 2007–2011" compares OSM and TomTom datasets.
Here is the conclusion of this article with aspects that I consider as particularly important in bold:

In this article, we outlined the development of Volunteered Geographic Information in Germany from 2007 to 2011, using the OpenStreetMap project as an example. Specifically, we considered the expansion of the total street network and the route network for car navigation. With a relative completeness comparison between the OSM database and TomTom‘s commercial dataset, we proved that OSM provides 27% more data within Germany with regard to the total street network and route information for pedestrians. On the contrary, OSM is still missing about 9% of data related to car navigation. According to our projection for the future, this discrepancy should disappear by the middle or end of 2012, and the OSM dataset for Germany should then feature a comparative route network for cars as provided by TomTom.
In addition to the route network comparisons, we conducted further analyses regarding topology errors and the completeness of street name information. The results showed that the OSM dataset is not flawless; however, the trend shows that the relative and absolute number of errors is decreasing. Thus, it can also be discerned that not only is new data being added to the project database but also quality assurance is becoming a major factor within the OSM community. Our findings with regard to turn restrictions within the OSM database, which are of critical importance to navigation, showed that based on the current development rate and activity, it will take more than five years for OSM to catch up with the information found in the proprietary dataset used in our analysis. This slower development in comparison to the regular street data collection can have several reasons. It can be based on the fact that turn restrictions cannot be seen in the regular OSM map and thus are less appealing for contributors to be added. Some members might also not be familiar with the importance of turn restrictions for the dataset or do not understand how to implement them correctly.
Overall, a certain trend can be distinguished from our studies, as well as in all other studies conducted to date for the countries that were examined. Preliminary statements and conclusions in the past were that OSM data is sufficient for use with map applications. Today we can say that, at least in countries in which the OSM project is well developed, the data is becoming comparable in quality to other geodata from commercial providers regarding the different factors analyzed in this paper such as temporal accuracy and geometric accuracy.
However, several questions remain and further research is still needed. One important factor that has not been addressed yet is the importance of whether users who contribute data to OSM should also maintain it. Also, it is unclear whether missing attribute information, such as street types or names, if added at a later date, could be analyzed and provided useful insights. So far it seems as if processing within the OSM project is closely related to visual factors, meaning that most data is collected in areas where there are white spots on the map, and thus no information is available. We will investigate specific questions regarding this user behavior in detail in the near future. It will be important to obtain further information on the project‘s participants and data contributors. These are some of the questions that need to be addressed: Are OSM mainly long-term contributors or are most of them so-called ―submarine users‖; that is, do they appear for a short period, add information, and then disappear again? Do members only add new data, or do they also edit existing information? Can an activity radius or area be determined for the participants of the project? Is the administrative area of an entire country completely covered by volunteers of the project or are data contributions by agencies playing a major role in certain areas ?
It will continue to be important to carry out studies about the quality assurance of VGI. Preliminary suggestions have been made on how consistency of compiled VGI data could be achieved by improving quality during production and providing quality metadata for the users [43].