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).
When it comes to Raster maps - there are plenty of choices, some better some worse but NONE of them will give u a...professional navigation if, for example, you are a taxi driver simply because they all 100% dependent on on-line navigation data even if you have some raster maps preloaded. (Especially frustrating when you travel abroad and have to change sim-cards(((()

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.