I've used Nokia Drive and Garmin on my HTC Trophy. The Nokia Drive is nice but, very basic in the navi setup. It does not have the advanced features as Garmin does.
I use Garmin 95% of the time. It finds what I need the best out of both of them. It does need signal, so some questionable areas, I see the main road but, not surrounding roads (as cell service was not great in the area).
If you have a Nokia device, then might as well try the Nokia Drive to see if it fits your needs. I perfer the Garmin, it just seems they have the experiance with these devices and it shows. The price is the problem with it being $30..
I do wish Tomtom came out for Windows Phone 7, That was my favorite on my WM6.5 devices. I would even give up a gig or so for maps...
I would prefer something that fully fits in with Windows phone 7, not just an app. I do wish Microsoft had the right to make a turn by turn navi... Both the Nokia and Garmin feel like what they are, 3rd party apps(nothing from contacts or other parts of the phone, to route to a contact, etc.).