MAPS.ME uses data from OpenStreetMap, meaning the data retrieved from that service is a community-based map. Anyone can freely edit and contribute to the map, a map concept that is similar to Wikipedia.
OSM provides a map editable by everyone; just like the web-based encyclopedia, Wikipedia. Using a community edited map allows one to correct isses they encounter. Combined with the monthly turn around, these edits are added for navigation quickly.
It can take some time for an area to be rendered. This tends not to be a problem during navigation since only a small area must be rendered as the map moves, and this area isn't immediately needed; though likely a bigger issue with smaller screens.
Offline access has been updated to include search and directions in areas that have been downloaded for offline access. The biggest size for any single offline area is 120,000 square kilometers, however, multiple areas can be saved. Offline access isn't available in some regions, however.
Map downloads are huge in size, ranging from a few hundred megabytes, to a couple of gigabytes, depending on the size of the map being downloaded. Luckily, the app can be set to store them on external SD memory.
Navmii GPS World (Navfree) has voice guided navigation for its maps, even offline so users do not need to mess around with the app while driving, which is safer than having to constantly look at ones device.
Offline GPS will tell you when you have left your tour (and approximately what direction it is in), but cannot re-route without internet connection. Which can be inconvenient for those that deviate or get lost.