This is an excellent plugin for those who know some CSS/HTML knowledge to set it up. For example with Photog there are two possible search boxes: one in the header of all pages and a second one displayed only in the content area of search pages that returned no results.
The first one has HTML/CSS class "s". By default, Search Autocomplete does not work with that. The second one has HTML/CSS id "s". This field gets autocompleted with the default settings. Both of them work after modifying the searchform.php and adding
class="s" to the search form and then changing
.s in plugin settings.
A possible way to make this easier for users is to query which forms they have on their pages and ask for which ones they want it enabled to. How much work this would be is hard to tell, but at the moment any user whose search form doesn't have id="s" may be flummoxed while trying to figure out what is a "jQuery selector". Telling it's the CSS/HTML #id or .class of the search box where id and class are substituted accordingly might help some users.
As an aside, why is there a two character limit to the autocomplete?