Hi Helen,
Are they useful? Well before we had a the C+C we had a mgf and it had a broken wind stop, we though we didn't need one, but eventually got one and it made such a difference. Our Micra didn't come with one but we bought it from the dealer at the same time so couldn't tell how effective they are on the micra as we have always had it on and not experienced it without. But from our experiences with the mgf it makes a big diff, its much quieter, but the micra generally is apart from the body shake with the top down, the best bit is the back of your neck is much warmer due to less wind trubulence.
If you would like some pics of it let me know and i will email you some.
Hope this helps
Kind regards
Mark