It depends. The GPS is only as good as the data it's been given. So I've found that for long trips, it's generally good, but it can make some really bad calls sometimes. For instance, I frequently drive about an hour north from my house to DC. My car's GPS (using HERE maps) gives me a completely sensible route, which I follow. Google Maps, however, wants me to take an early left turn onto some windy little single-lane country road, probably because it might technically be 100 feet shorter in absolute distance that way. But it's a much slower route: I tried it once or twice and got stuck behind very slow drivers. I never went that way again because the slightly longer route is along main roads and doesn't have this problem.
Also, very close to your destination, GPS can make errors. I'm thinking of one restaurant I used to frequent, where Google Maps would tell me to turn before the restaurant and go an extra half-mile in a big circle, all because it didn't think I could take a left turn into the restaurant's parking lot, when in fact there's a turn lane there for that very purpose.
Basically, with GPS, you need to zoom out and look at the route it's chosen for you, and make sure it isn't doing anything really stupid. And if you're not familiar with an area, you need to be extra cautious because it'll happily guide you onto small residential streets or other stupid routes. It also helps to have multiple GPS units running at once. My car's system works pretty well and of course is well-integrated, but it doesn't have traffic updates or show alternate routes in real-time (it's based on stored maps). Google Maps does those things, but more frequently makes poor choices for routes (tiny country roads like I mentioned above). Having two different systems in parallel can help you cross check them against each other. The bottom line: never fully trust a GPS system.
I really should install Waze and try that out to see how it compares to Google Maps.