Pros and cons of off-market "pay-per-lead" services? Success with any companies?

[Michael Mahbobian]

My partners and I started a company that specializes in generating motivated-seller leads for investors. We're interested in hearing investors' opinions and experiences, as far as using pay-per-lead as a way to supplement their deal pipeline (including success stories, negative experiences, recommended companies, pros, cons, etc.). What would the "ideal" lead product look like for you? Thanks!