The built in GPS in my wife's Mercedes is terrible. It's like a 1999 GPS in a 2011 model car.
My Magellan impressed me today. I was driving a route that I do probably once a month. I let the GPS navigate just so I could play with it a little bit. When I got close to where I wanted to do it wanted me to take an exit that I didn't think was correct as I had never gone that way before. I decided to trust it and took the exit anyway. Then it wanted me to take a left turn on another road that just didn't look right, but I was in no hurry so I decided to see where it would take me. The route I'm used to taking is going from one freeway to another and I normally have to take a surface street that runs through a business district and has about 8 traffic signals to go through. This road the GPS took me on had a 45 mph speed limit and one stop sign before it got me to the other freeway. I had no idea it was even there. The area has undergone a great deal of construction over the last few years and I had no idea this road was there, but the GPS knew it was there and knew it was a short cut.