I had a fun experience in the Dominican Republic once:  There was no official coverage of the island at that time, but I was able to get a map for my Garmin from some shady Russian website.  The roads were accurate for the most part, but they were lacking all of the metadata like highway size, speed, direction, etc.  We used it to drive from our resort to another town down the coast, and lacking all of that data, the Garmin just routed us through the shortest path, which turned out to be a country road out in the hills that eventually disappeared.  Luckily I was able to ask some bemused farmers how to get to the right road - basically drive 20 miles back and get on the highway.