A recent Nielsen report, The Power of Defaults, studied 69 search engine users for a variety of tasks and came away with some interesting data (that's already being reported at most major search blogs). What's interesting is a discussion that Ammon Johns started at Cre8asite.

Ammon has some good comments:

I think this is showing that there are of course a percentage of users who simply click the top result, but that these are not a majority. From this study it would seem that a third of people were clicking the topmost result even when it was not the one the engine had truly placed top. The other two-thirds of users were obviously applying judgement on which listing to select, and not all were even staying within the top three results.

So does Kim:

Search results can show or validate whether or not someone made the correct query. This can be determined by the results themselves and how or if they

a - relate to the query
b - satisfy the query
c - meet subtle requirements of the query
d - meet the needs of the person who made the query (for example, if they like sentences that make sense vs. descriptions that pull key words and toss them together like a salad

If the SERPS don't meet a - d, a change in the query likely follows by the end user. Whether or not the first search result was the "best" choice isn't or may not be, the decision maker.

Chime in - I think there's more to this study (including a lack of demographics) than meets the eye.