I've used the services of Doorknockers extensively and I don't have a bad word to say about them.
You really can't go wrong. You stipulate the amount you want per house and the type of house you want. IThey work exactly to your requirements ... if ever they stray, they refund you!
On the odd occasion (literally 2or3 times) I've decided that a house was priced too cheaply and 'refused' it. Paul honours his word and gives you a refund.
In my experience, the work they bring on is normally at a higher rate than I would go for on the door. This is because they're experts at what they do and know what rate they can realistically get.
Obviously in a team of 5 canvassers, there might be one salesman who's had a lean evening and decides to go in slightly lower to win the deal. But as I said, this has happened to me only once or twice and is always resolved by Paul instantly.
If you're thinking of trying them out, ask them over for a Sample Day - what have you got to lose??