I got a cheapo set from the bay and they worked just fine. It was a 4 sensor set with led read out.
Bit of a pain to fit as had to pull the rear number off and drill through as well as colour match the sensors themselves. Suppose it was worth it in the end as it looked factory.
Personally I would prefer to go with a camera as less hassle to fit and you can often get a number plate surround that includes the camera which then links to a rear view mirror with a screen in it.
How well has the paint lasted on the sensors? I've been looking at some of the cheap sets on the bay as well, as most seem to be sub £20. Whilst they do provide coloured sensors, i'm doubtful these will match the colour of the car very well, and may look very obvious.