I got a GPS when my phone carrier started charging for voice GPS. And as mentioned above, the screen is too small to really see the details and I certainly wasn't going to pay for the ability to add voice directions.
My phone's screen also fades out after a few minutes if I don't touch the screen or a button - so while driving, it can be a pain.
But I've never been a fan of multi-use electronics. If you use your cell phone for music, GPS, camera, and phone, and the screen goes bad - you have nothing. With a separate GPS, mp3, phone and camera, if the screen on the phone goes bad, I can still use all the other functions.