Ask YC: Ever used an SEM aggregate technology like Clickable? Recommendations?

I am looking into a simple way to manage my SEM efforts. I ran across a company called Clickable (http://www.clickable.com). They charge for their service using percentages of revenue you plan to spend on the ad campaigns (ie: if you spend $1,000 you get charged $29.99). I am not sure I love that ratio, do you guys recommend others? Have you used this? Is it worth it?