I have two people who leaflet for me. They cost much more than shared delivery, so I keep an eye out for areas where we get good responses (perhaps someone's retired/disappeared) and they blitz there. As an example there's an area locally where we're getting about three times the normal response rate, which outweighs the extra cost of our chaps; they're going to go round in circles in the same area until we stop having the results.
The bulk of our leaflets go out with a local magazine because finding and organising people who will do the job in the right way is not too easy. The ones I have are diamonds; others have sucked.
Vin