I have 3 issues with using Smartphones for Turn-for-Turn Navigation:

First is that I think GPS devices are better and faster in terms of getting your actual location. While some phones are better than others, most are still I feel not at par with dedicated GPS devices.

Second is that most of the software available for Smartphones are paid applications. Some of these applications do have trial periods, but in the end you will have to pay for them.

Third is that the lack of a really good software available for Smartphones for navigation. Those that are available I feel are not as good in terms of user interface and friendliness as the Garmin software, and not as easy to update the map on your own.

In the end, I think dedicated GPS devices are still better. But then if you don't do a lot of travelling out of town, then you won't get to use it as much, hence that is the issue.