I used radar on my first patrol, and it brought planes to me like bees to honey. Radar detector however works GREAT,

It has saved my bacon several times in the past.
As for sonar , I have used it to get range info on non escorted merchants in very bad weather.
I'm on my 4th boat now and I did not bother to waste renown on radar. I'm also finding out the anti radar coating for the snorkel does not work that well either. Not for what it cost in renown.