I've been using it as my sole navigation GPS since I got it (replacing my Garmin 60CSx that was my combination navigation/hiking/biking/geocaching GPS).
It works well, although is somewhat worse at directions than the Garmin, as far as taking me what I think is the best way to go, but still gets me there.
I've been using Google Navigation, which does voice with street names, although the voice is irritating (would like some different options such as a sexy British accent) and she can't pronounce anything correctly it seems: For example "Colebrook" becomes "Cawl-ee-brook" and "Sunnyburn" becomes "Soon-eee-burn"
But it does do a bit that my standalone didn't (keeping in mind I don't have a "car" GPS but have one for hiking). The voice directions are nice, as opposed to the "beeps" my old one did when turns approached. Google Maps, as well as POIs are updated more often than the Garmin maps were (and I had to buy updated versions of Garmin maps). It does traffic and satellite overlays as well, and alternative routes which my old one didn't. Also does Google Street View for the next turn...not really handy but is a nice "show off" feature. But it does NOT do Topo, as far as I can tell (which is nice to have when I'm hiking/biking)
For the first time yesterday, it actually crashed on me, which was annoying since I was somewhere I wasn't familiar with....I actually had to reboot the phone and by the time I got it back up, I was about a mile off course. That may have had something to do with the update, perhaps? I'm hoping it doesn't happen again.
My other concern with it is while the GPS functions off of satellite, the maps function off of Google Maps...so no cell service = no maps. I haven't ran into this yet, and I think it caches some of your route, but could see it being an issue when driving through areas with long periods of no service. I will actually know more tomorrow since I am going on a ~300 mile trip to my parents for the weekend and will go through about a 30 mile section where I know the signal will be bad.
I have not tried it for geocaching yet....haven't downloaded any apps, and it appears the best one (C-Geo) is violating Geocaching.com's TOS and I don't want to support it. Supposed to be an official one coming soon, but in the meantime will probably try out Geobeagle or something. Also not sure I will want to be holding my phone while I'm traversing streams and rock fields and running away from rattelsnakes, so I may stick to my 60CSx anyways for that.
I'll probably use a combination of both....the phone/Google Nav in the car and then my rugged 60CSx for hiking/biking/caching (especially since I already have belt and handlebar clips for it) and to serve as a backup if the signal fails on the phone.