I don't have an iphone, but the Google Maps app on Android phones has a really good navigation feature. I also have a Gamin, and just finished the installation of an OEM navigation radio in my truck. I love the OEM navigation more than the others. I like the Gamin a lot. I just don't use it anymore. It's very easy to use, and I love that it'll show you how fast you are going and what the speed limit is. As for my phone, the only problem I have is that I don't have a phone holder in my truck. I never to keep looking down. Plus, it drains the battery pretty fast. But, I can charge it and plug it into the radio simultaneously. It will talk through the speakers, but the radio must be on auxiliary. That means no music unless I use the media player app at the same time. The directions will dim the music automatically so that I can hear the navigation instructions.
As for navigation problems, I have found user error is normally my problem. I just go into the menu or options and change the settings for how I want the route to take me.