Well, I'm from Europe* and I went to South Africa last month (for World Cup). I tried GPS navigation of my N900 and was extremely happy with it. For two weeks, I relied solely on OVI navigation and I haven't missed one single turn! (I’ve made almost 3.000km.) I was impressed by how it was able to find directions to every single place, even to one of my guest houses in some little town very far from any major city.
* maybe there are some differences to US version

There was just one single small mistake (London Rd in Cape Town is marked one-way in the opposite direction).

IMHO, GPS navigation is one of best things of N900 (and it's a great phone)!

P.S.: But it wasn’t able to find my location while traveling with airplane, 30.000ft above Africa. Anybody knows why?