I also fitted just the fronts, at a million pounds a pair I don't need rears, the dirt can just spray on the car behind
As stated above, they are dead easy to fit with a stubby screwdriver (I did mine in the dark by torchlight), but could do with being a lot bigger, as there is still a lot of mud and grit getting on my lower door panels.
It takes a little longer than you think, as you need to clean where they will be fitting.
Even small flaps can help protect the lower paintwork on the doors a little, especially with the amount of stones and other crud thrown up from pot holes these days.
I agree about the rear flaps being a virtual waste of time though, and if longer and made of the same rigid material they would break off really quickly at the first downing off a kerb.
Follow up to my last, I fitted flaps to the back wheels, could not get the front wheels off even with all four bolts undone on my 2013 Note Ntec+ auto 1.6. I had only bought it and I had to go back to the garage to get the steering wheel lined up and tracking sorted, so I got them to fit the front ones when I was there. The service manager told me he had spoken to the sales manager as he was not happy with the tyres on the car which had been sold only a week before. Result three new tyres at no charge to me !!!