I'm really getting into the iPhone idea. I have multiple GPS softwares and each one works better for a specific scenario.
When I want to upload someone else's GPX file for a specific ride I use GPS Drive. It's great and you can upload the entire map so data roaming isn't an issue.
In conjunction with the iPhone Greatest Ride App is great. Greatest Ride is an app that let's people upload their routes. So wherever you are you can pick some great routes that other people uploaded and export the GPX file into GPS Drive.
And then there's Google Maps and iPhone Maps for city driving where data is not a concern.
Tom Tom is also pretty good as long as you want to get from point A to point B. All maps are loaded on Tom Tom as well so data is not used.
With all of this I find the iPhone hard to beat, especially with my blue tooth setup, MP4 player, telephone, radio, etc.
Pretty much an all in one device.