Pay-for-performance is an extremely popular model in marketing today. It's the natural consequence of the extreme price competition among solution providers
But it's not without risk, as illustrated by the case of a company that had contracted with a pay-per-lead vendor to generate appointments for their field sales team.
The Rest of the Story...
The problem with pay-per-lead is that, if the vendor can't generate good leads then they'll generate bad ones. So while you can comfort yourself that you won't pay for them if they're no good, it's always too late.
It wasn't until two of their best salespeople quit that the company finally figured out that wasting a rep's time is worse than actually paying a fair price for marketing work.