A recent post by Chris Smith mentions the difference between being in first and being in second on Google for a keyword. In his case, the difference in traffic was as much as 60%!
I can tell you that our site receives approximately 30k of visits on average per day from Google, just from keyword searches for Term X. There’s typically one or two Sponsored Links just above us on the SERP, and a few Sponsored Links on the right side column, too.
When we dropped to second slot on the SERP for Term X, we lost approx 18k of visits per day. So, there it is: the difference between the number one slot and the number two slot for a major keyword term comes to about a 60% change in visits!
Read more at Natural Search Blog.