I have an Android phone with the Copilot GPS application, and previously used the Sygic GPS application.
With smartphones, you have to have some sort of suitable docking station or mount to hold them safely in view of the driver, and you have to have provisions for 12v
power. Some of the mounts are expensive enough (depends on your phone) to make that a consideration.
Most phones cannot walk and chew gum at the same time so if a call comes in you may lose navigation or need a lot of key clicks (=eyes off the road) to maintain both. Similarly if you use your phone as an MP3 player on the road you may find that switching back and forth isn't convenient.
I'm thinking I'll switch to a dash-mount 2xDIN GPS like the one upthread at some point.