Orders of magnitude better for navigation. Far more up to date data, faster, speed limits on it, proper accurate turn by turn and its 100% stand alone without a network connection for all features. I did try google maps on android but it was literally littered with mistakes, lag and shoots itself regularly.
Only thing it doesn't have is street view at destination but that's pretty useless to be honest at least in the UK as most of my destinations have no data availability.
You will only get this on a Nokia handset. The build in Bing maps is crap. I've got a lumia 710 and an 820 at hand (I'm testing the latter before i rely on it) and both map solutions are the same. You can get the 710 for literally nothing now - less than a galaxy ace, which is a hunk of crap compared to the 710.
I've used Nokia maps back in the Nokia 6303 days and its always been good. This is just a winphone port.
The killer app for me is Nokia drive. There is literally nothing better out there.