My GPS (Tomtom) and I have a love hate relationship. It loves to take me the wrong way and I hate it in almost every way.
In California, it once took me through and apartment building's parking lot to get to a Wal-Mart.
Another time it had me turn off a high-speed, 4 lane road and took my up a one-lane mountain road at 10 to 20 miles an hour to save 1/8 of a mile. It took me 20 minutes to save that 1/8 mile.
Once I was making one of my twice-a-year trips to Dallas; It told me to take an alternate rote to save 2 hours. I knew better, but tried it anyway; it took me an extra three hours to get there and there were no detours or anything like that!
I have tried every optimization setting it has and I still can't rely fully on that thing! I have found I always have to check the map before I listen to that soft, sweet, British, female voice when it says, "Turn right ahead... you can trust me, go ahead, turn..."