the answer is probably that it depends where you live and want to travel. I have used one (griffin I think) to go to work, it is ok.
We have travelled to Wales with it (not in a Clio), and it was fine all the way, we only retuned once.
As soon as you hit the M25, doesn't work anymore.
Tried through France, couldn't get a signal for more than a few minutes.
So overall, I would class them as not great.
Now invested in a proper ipod connection for our family bus, works way better and the sound quality is also much better (plus I have some control on the steering wheel). Cost £80 plus about 1 hour of my own time to fit.