Your buying the App for what, £60? £160 for a Tomtom that you will have you everyday of the week and will work perfect everytime is a bargain.
You can buy Tomtom's for like £300 these days that do all of the same things that the iPhone App does.
Personally I think you'll regret it if you get another one. I used my iPhone 4 without the mount the other day and over a 20 minute journey it zapped the charge from 80% to 30%-or-so. The Tomtom App uses LOADS of battery life, you'll find yourself struggling for charge if you go anywhere but the corner shops with it on tbh. lol.