I have them and I used to live in an area where reverse parking was always required. As a conseqence consider myself an expert and sometimes able "speed Parker" :-)
Still I've come to like them and I think they work very well :-) snot just a out parking andnot hitting things rots about leaving the standard space between you and the muppet behind so they don't hit you on the way out ! :-) like them have them wouldipa £300 for them? Yes now having used them for a year :-)