I recently used my iPhone as a GPS with Navigon (paid $49.00 for the app with maps of NA) while my Zumo was being refurbished, with a handlebar RAM mount. Works great. With it's ability to find points of interest using Google (when there is 3G or WiFi coverage) its capabilities even exceed those of the Zumo. Even without Google available there is a limited POI database. There is real time traffic data available for an extra charge. I don't have it.
The screen is small, but bright. It is only visible when mounted on handlebars near me, not up front and high where I like my GPS for road visibility.
I have not had any problem with keeping the battery charged using 12V bike power thru a car charger in my tank bag. It only overheated once when I put the phone under my tank bag clear plastic map holder on a very sunny day. Mounted in the RAM mount on handlebars no problem with heat.
By itself it is not waterproof, or very bulletproof, so I wouldn't want to leave it handing out there unprotected for long, as it costs up the wazzoo.
I used it to route and could easily hear route commands with a wired headphone set, and with bluetooth connection. I use the Camos BTS 200 bluetooth headsets, which can connect to my Zumo, my iPhone or both. I also have wired earplug speakers, but I like the Bluetooth better.
As a bonus I had thousands of MP3's for almost unlimited music, and could easily review my emails, texts and voicemails when stopped.
I also have a number of weather apps so I could see live radar maps and avoid storms. Oh, I was also able to get and make phone calls!
I have my Zumo back, and use it as primary GPS, but I will retain the iPhone (iPhone 4) as a backup and for supplemental info.
Opinion: It seems to me that Garmin will have to step up their units capabilities to match what these smartphone units can do if they are to stay competitive, as I wouldn't pay $800 for a Zumo 665 when I can duplicate and exceed with a phone as soon as a good waterproof/bombproof case comes to market.