Oconnom - I don't know if you've actually used that type of sensor before or not. I've had it on 3 cars (2 fitted professionally for me and the third I did for my dad, fitting it myself). My experience was that it's not bad and is certainly invisible, BUT it's difficult to get the length right so it doesn't read the side of the car. They can be erratic and tend to be over sensitive, they don't like being on for any length of time (while sitting in reverse waiting for someone to get out of a parking space) and on one occasion, mine beeped merrily away - not with the solid 'stop' tone - as you'd expect, and continued to do so right up to the time when I gently backed into a rough brick wall .

I was going very, very slowly and it was raining, which may or may not have had anything to do with it, but I was relying on it, it let me down and left me with a scratched bumper. The inescapable fact is that if was really a good system, manufacturers would use it at the factory, but they don't as it's affected by the amount of fuel in your tank, the metal structure inside your bumpers etc.

Have you thought about the Cobra R0394 flush ones (such as fitted to Audi and BMW). I've fitted those to two cars in the family. They look great and work well. They are a bit more of a faff to fit (but not that much) and you have to buy the correct hole saw, but they are much, much better in use. I have a set purchased for the XF too, but just haven't got round to fitting them yet.

Good luck, whichever you choose and I hope the above is of some help.