Computers don't think like human brains do. If it was set for shortest rather than fastest or if it did not consider traffic, it makes what we feel is a dumb decision.
I had mine on going through an area I'm familiar with. The best way was to continue on the road to the light, make a left and destination is a couple of miles up the road. Simple enough. The GPS though, calculated a shorter route. Curious, I followed it. It had me make a left a quarter mile sooner, a right, left right, left, like a staircase. Sure, it would get me there, save probably 100 feet, but what a PITA.
Next week I'm going to two places in New York, about 140 miles from me. Both addresses are already programed in, but I won't turn it on until I'm about 5 miles from my destination. I know how to get to the city, just not the particular street I need. Great toy to have on a trip though.
Even better to have in Europe. Street signs are not easily found in some places and you have to be able to spot street names like Via Digiacimoluigefussillipastfagoola on the side of a building on the corner. Easier to follow the arrow showing a turn at the next intersection.