Have you used Ovi Maps on other Nokia devices, such as E71/E72/E75? It has good turn-by-turn navigation, free worldwide maps since Jan 2010, and you can pre-load maps (and use it in a place you haven't been before, without a data connection).

I haven't used iPhone or Android much, but I believe Google Maps doesn't have this on either of those.

So, if your experience with Ovi Maps is only on N900 ... I think it would be better and easier to get a more complete Ovi Maps (assuming Nokia is porting it to Qt for Symbian, it should be quite an easy port to Meego, which should be compatible enough with Maemo to make it a no-brainer).