I've never used any apps for an iPhone since I've never owned one, but I used to use CoPilot for navigating fixed routes and that always worked pretty well. Whether or not its better than the equivalent TomTom or Garmin apps I couldn't say though.
Ridewithgps app? Plot the route on your pc, then use the “navigate” function once you have the route loaded in the app. I think this might now be a premium feature for about £6 a month.
Is there a navigation application that allows you to specify waypoints, for the iPhone, that people have used and can recommend?