Thought about getting one for my Clio 172 a few times. One thing you have to watch is water getting inside and then the leading edge of the bonnet just sits in the water. They are a great idea but that is something to watch if you do get one.
Unless you get one that covers the entire front of the car, you`ll still suffer stone chips. The only benefit is that a stone chip on plastic bumper won`t rust like on a bonnet.
To be honest a full bumper bra is the only thing that will 100% protect the front end of your car from stone chips. I bought this for my inferno 225 for the purpose of protecting it should I take it to a show.
A bonnet bra will protect the bonnet although with the Megane and Twingo having a bumper that comes up to meet the bonnet at the front it's not really that exposed anyway.
I actually think they can look quite cool and break up the colour on the right car, this was the first on I bought -