I didn't think Sprint Navigation was useless, but it definitely lagged a bit when recalculating or refreshing the map view when making multiple turns in succession. It was a decent navigation option, but when I knew ahead of time that I was making a trip in to the unknown, I always made sure to grab the dedicated GPS device from my wife's car.
I updated my Moment to 2.1 from 1.5 on Friday, and yesterday I drove to an unknown place in an unfamiliar city. I brought the dedicated GPS with me, but decided to give Google Navigation a chance. Compared to Sprint Navigation, Google Navigation is a champ. Recalculating was quick, view updates were quick, and its integration in to the rest of the phone is much nicer.
I'm not sure what happens with Google Navigation when the phone is out of data range. The dedicated GPS device obviously has its maps stored locally so as long as there is GPS, the unit is usable. With Sprint Navigation, when you were out of data range but still had a GPS connection, you no longer got live map views, but still got directions, distances, etc, so it was still usable. I was in data range all day yesterday, so I could not test that out.
There are two things I did not like about Google Navigation compared to Sprint Navigation. At least by default, Google Navigation uses minutes as measurements far more than Sprint Navigation (ie, go on this road for 43 minutes), where as Sprint Navigation uses miles as measurements. The only time where I care about minutes, Google uses raw minutes while Sprint Navigation tells you what time you will arrive (Google: you will arive in an hour and 6 minutes; Sprint: you will arrive at 1:08PM). The other thing I like about Sprint over Google is when an audio direction runs too long, Sprint will cut it off and start with the next direction, whereas Google will let the original direction finish and then move on to the next direction even if its already past. You need to make sure you are looking at the map so you don't miss a direction.
Anyway, these are my findings after one day of Google Navigation use. Long story short...I purchased a stand-alone GPS device because I felt Sprint Navigation was not up-to-par. Had Google Navigation been on my phone from the start, I don't know if I would have made that separate purchase.