In a keyword search, you can fully control the Ajax search. In other words, what you want to be searched for is exactly what the search will search for. Whether it will be possible to search for ad descriptions, only promotional ads as well as a lot of other settings is the decision of the site owner.
Loading results is fast and efficient. It is also smart because it already loads the results and suggestions live when entering the first letters of the search.
Besides you can monetize with promotions inside of the keyword suggestion promoted listings.