One of the search terms I focus on has recently gone from 4th place in Google to 1st place.
This has resulted in a doubling of site visitors due to that key phrase!
I have had 4th place for some months and have recorded the recent change over a period of 1 month. I have also got an adwords for the search term and have actually seen a marginal drop in the total numbers doing a search for that term over the last few months so the relative increase in site vistors is greater.
Incidentally, taking the adwords stats which shows the total numbers doing a search on that particular search term and my figures showing success from being 1st place, my site now gets 1/3 of all people carrying out that search!