Kent Lewis from Anvil Media responds to an earlier post about selecting an SEO company:
"I love questions like this, as infrequent as they are these days. Do PR agencies asked to get paid by the column inches of ink? Do ad agencies get paid based on resulting sales? Why should SEM firms have to work against a different standard?
Any good SEM professional is a marketer first and foremost. Unfortunately, like many other forms of marketing, we are limited by the client or company's infrastructure, as well as outside influences like competition and search engine algorithms.
Assuming we could work in a vacuum, with complete control over product, service, placement and the entire sales process, I might entertain the idea of pay-for-performance. That has never been possible in my situation.
That said, there are ways to evaluate a good SEM or SEO vendor, but I encourage you not to start by looking at rankings. Look at reputation and results, with sites like SEMCompare.com. Look at the visibility of clients, case studies and testimonials.
I've outlined ways to evluate an SEM vendor in this article in the Resources section of our site."