Not too bad
Installed this on an N95. Maps are OK, performance is OK, routing is quite good too.
Shame the satellite imagery seems to all be the baseline default stuff, rather than the normal Google Earth/Maps multi-resolution ones, but that should improve. Not worth bothering with at the moment.
The controls could be easier too, but that probably depends on the handset. Usable as is I guess.
GPS integration would be nice, probably difficult to implement though.
Traffic data will be useful, could be one big reason for using this alongside the Nokia navigation.
Does chew through a bit of data, but that's what flat-rate is for!