OpenStreetMap has such a level of detail (at least in Europe) that I can completely relay on it for my trips.
I went on a drive from Italy to Holland a month ago using MapFactor Navigatior with OpenStreetMap maps, and I was really surprised how good it worked. It even had POIs for the toilets on the highway! Amsterdam was mapped really well as well, with house numbers and everything.
And it was completely offline (even the routing algorithm) and free.
Depends on the place. In some places, the OSM is better than most commercial providers (e.g. Balkan in Europe), in others it is good enough. In Thailand, I got around by using OSM maps; I didn't have neither commercial maps nor connectivity, co I couldn't compare it to other maps.
It's just a pity the apps aren't quite there on Android. I find MapFactor Navigator to be clunky and outright confusing, Telenav Scout is only available in the US, Skobbler seems to have been withdrawn (at least, it's not accessible in the UK Play Store), and OSMAnd is great for what it is, but has no traffic information, and is a bit guileless in selecting a route - certainly enough so it's not sensible to trust it for long journeys.
Yes, I was hoping to use that app- I can see that in the US webstore, but nothing comes up searching for it on my UK phone.
Maybe they're half-way in transitioning from Skobbler to Scout, and they've removed the former without adding the latter. But I'm pretty certain it's been like this for weeks if not months, because I've looked unsuccessfully before.
I'm from Romania, so it isn't just in the US. Maybe the app has problems in the UK that need to be fixed, as your zip codes are nonnumerical and you're driving on the wrong side of the road :-)
They had an update in September, so it's alive. You could contact them at support@skobbler.com … BTW, I care about them because the company may have been German but the developers were Romanians :)
From my experience OsmAnd routes equally well or better than other applications. I've only once caught it giving me suboptimal directions, our old TomTom has a much longer history of weird moves. Google Maps seems to do as well as OsmAnd, though I hardly ever use it.
And it was completely offline (even the routing algorithm) and free.