I'm not a traveling salesperson so I don't need an app to be flawless, and overall, I'm happy with Google Nav. However, having had the opportunity to test it side-by-side with a Garmin Nuvi 255w, I will say that the Garmin still has the edge, at least in my neck of the woods. A couple of reasons I will still take the Garmin on long road trips:
1) The Garmin has yet to miss a pronunciation on any street we've been on. Google routinely mispronounces street names. Not critical, but annoying when another device seems to have already mastered it.
2) Perhaps because of the mispronunciations in #1, there are several streets I can't navigate to using voice search on the Droid. The voice search works great if I want to go to McDonalds or BestBuy, but if I'm trying to get to a person's home or a small business, the voice interface rarely understands me. (In fairness, the Nuvi doesn't offer this service so I suppose it's unfair to hold this fault against Google.)
3) Google tends to assume that any street that borders a location is a satisfactory end point. I live on a neighborhood cul-de-sac that backs up to a major street. When my home is the start point, Google knows to take me out through my neighborhood, but when returning, it assumes I can just pull off on the side of the major road and climb over my backyard fence!
4) God forbid your route takes you on a road with a long name. The Garmin will tell you to take "Exit 16, Sunset Rd." Google will refer to the same exit as "Exit 16 A-B, west on Sunset Road, State Road 1536." I didn't know Sunset Rd had a state road number, and I certainly wouldn't know where to find it to confirm my heading!
5) Through no fault of Google, there are many areas I travel with poor 3G coverage. If I begin my route in one of these places, Google never has an opportunity to cache the necessary maps. Garmin has never failed to pick up a satellite from any location.
On the plus side, Google's maps seem to be more accurate. The Garmin device has not been updated for new roadwork around us that has been open for about 6 months. Google has been flawless in these situations.
If I didn't have a GPS device, I certainly wouldn't run out to get one as Google's offering is adequate for most situations, but since I do have one, I think I'll continue to rely on it for the time being.