A family trip this weekend reiterated for me why I do not think that cell phone based GPSes are viable 100% replacements for dedicated GPSes in your car. At least, not yet.
We went up to Sullivan County, NY visit with some of my family. The route we take to get there passes through NE PA, including the Delaware Water Gap. My cousin, who's house we went to, lives in a rural area in which cell phone signal is spotty and often depends on how you orient your phone.
For navigation we had directions but also brought my Garmin Nuvi 200W, and both my wife and I have Android-based smartphones (a Droid and a Droid 2) with GPS integrated with Google Maps. The Droids use the Verizon 3G data link to download maps on the fly. In contrast, the Garmin stores maps locally.
We had no or poor cell phone signal, or no 3G for a good part of the drive. Notably, the times when we had no signal were the times when the GPS was most needed. I.e., rural back roads with poor signage and no street lighting. If we had needed to depend on a phone-based GPS we'd have been out of luck.
There are a few applications designed to allow you to download and store maps offline in case you lose your cell phone signal, e.g., Maps (-). However, this may not help you if you need to significantly deviate from your route, or if you need the GPS in an emergency and the cell network is down.
If your cell phone GPS usage is limited to areas with good cell coverage and don't consider your GPS unit as part of your emergency preps, then you don't need a standalone unit. As for me, I'll be updating the maps in my Garmin soon.