The 'roc is not a long car, so parking it is pretty easy. However the sensors can do nothing but help you. I've got an unwritten rule as to where I park my car in my work car park. I like to tuck it into the corner as tight as I can out the way. Everyone knows thats where I park, so everyone leaves me the space to park there. The sensors are always handy when parking up as I know I've got about a foot or two left when the tone becomes constant.
They are useful, and I'm glad I specced them, but I wouldnt miss them if I didnt have them as I'm a pretty good parker-upper.
I got factory sensors and have the onscreen display, but can't really say i look at the screen when i'm going backwards! Not a massive miss if you don't have the screen display, although it's always nice to have as many features as possible