Just trying to rival with Google.
I've only heard one negative opinion of Google in... since I can remember. It was something along the lines of the search results getting so watered down with people who pay to have their results higher up in the list that they're often invalid. Well... I have never had a problem with finding what I want on Google (unless I was to blame for searching on some obscure topic), so I'm not going to even slightly agree with that opinion, though it does seem somewhat logical.
All the ads people pay Google to show are either in a blue box at the top of the results, or in a separate list to the right. Either way, both are clearly marked with "sponsored link". It's engines like Yahoo, which merges sponsored and crawled links into one single list, that should die and explode.