If you're going to use your smartphone as a GPS, you need a way to mount it in your car so that you can see and hear the directions and still keep it just enough out of reach that you're not tempting to check your email at a stoplight. The Grab-It! Universal Mount gets the job done for a reasonable price.
The device comes with two suction cups. One attaches to your window (0r other flat surface) and the other to your smartphone. It's only going to work on a perfectly smooth surface. You can't attach it to a textured dashboard and your phone's going to have to come out of its case.
You might have seen these advertised on late-night TV. The plastic is reasonably good quality and the suction cups show no signs of deterioration after a couple of months of steady use. A couple of months may not seem like a long time, but anyone who's tried a lot of bargain electronics accessories can testify that they usually start falling apart right away. The Grab-It! Universal Mounts been available at $14.99 for a pair from time to time online. They're a real bargain at that price and they're still a pretty good deal at $29.99 for the pair.