First, let's define the navigation types: There is vector maps navigation and raster maps navigation. Vector maps ("artificial"): for n900 unfortunately we can only have Sygic and very, very sucking OVI (sometimes I even forget it's there).
(...) So, the answer is simple - if you need a professional and reliable navigation soft on N900 go for Sygic. Preload maps and POI of the location where you are now or will be in future and forget what the words "trouble" and "frustration" are.