I had the same question. I've heard people tell me that they avoid clicking on the #1 spot. Some of the rationale was that the #1 spot is sometimes too SEO'ed and thus not as relevant. Another reason given was the searchers like to skim the top 10 listings from top down to compare the summaries, URL's, titles and then end up clicking on something towards the bottom.
I personally look at title and URL relevance. If I see a URL I don't expect the correct information to be at, then I skip it until I find something more intuitive.
Anyone else? How does everyone else here choose which results get clicked on?