I don't have an iPhone, but do have a smartphone with GPS. Once there is a GPS lock, the phone works well. I also like the traffic reports and accident identification. I saved hours once because my phone GPS system routed me around an accident that had a 5 mile backup. You can get traffic notification with the Garmin, but its another $50 add-on.
The other nice thing about the GPS in your phone is what it adds to your pictures. WIth each picture you take, the geolocation (latitude and longitude) is saved. This information can be read by you Windows or Mac PC's. This is great for documenting your trip!
So even if an iPhone is too expensive for you, most lower cost smartphones will have the same features. Garmin is great, but the option you listed here is a good one.