I don't use my GPS much around Baton Rouge because I pretty much know where everything is where I go. When I had a job traveling around the country I quickly realized I needed one when I was in cities where I had never been before. I bought a Tom Tom at a Walmart in the town just south of Oklahoma City. I think it might have been Moore. Without it I might still be driving around looking for my hotel in OKC. It worked great on the roads but sometimes it made me take turns before my real exit. When I got to Little Rock my boss asked me to go to a Walmart for him. It was nighttime. The first Walmart my Tom Tom found was closed down. While driving to the second it ended up taking me around and around a cloverleaf. Overall it was better to have it than not to have it. I paid about $100 for it. In Boston somebody broke into my truck one night at the hotel and stole it. I drove to Baton Rouge without one and if I had had it I might have cut a few miles off the trip. When I got to Houston I realized I needed another one even though I am pretty familiar with the Houston area. I got one just like the one that was ripped off for $40 in a pawn shop and it was the same model and everything. It work fine just about everywhere except in the Chesapeake Bay area there are a lot of tunnels. Whenever I would go through a tunnel the GPS would lose connection with the satellite. Good thing I couldn't get lost in a tunnel.
Popular car GPS are accurate to about 30 feet. Professional GPS used by surveyors have millimeter accuracy. But you have to set them on a tripod and let them collect signals for a long time.