Ive got a TV with built in freeview in my T4 camper van, was 100 quid in halfords originally, but was an ex display one so the mrs got it for 25 quid.
Quite often watch it of an evening if I am kipping in the van, but cant really see much use for one in a car TBH, unless for some reason you have to spend a lot of time sat stationary in your car (waiting for lots of ferries I guess, or if you are a taxi driver)
Careful of your battery if the engine isnt running, I have a couple of batteries in the back of my van that it runs off that then charge when the engine is running, which means I cant ever accidentally flatten the starter battery!