If you're going to offer a search field for users to type into, it has to help out the user. The biggest danger of using free text search is that once you have it people can type pretty much anything in there (and they will). This can result in lots of null results screens, which is a frustrating experience for the user. To prevent this your search field must incorporate auto-complete and auto-suggest.
Auto-complete is when the field shows search terms that could be formed from the letters the user has started to type. This is particularly useful when you sell a limited set of things and you need to guide users to the right search terms.
Auto-suggest is when the field gives the user similar searches to the terms they have entered, or searches that are popular on the site and you know most users make. You can also suggest individual products in order to direct the user straight to those product pages and help speed up the searching process. Between these options you can guide the user away from dead-end searches and also help educate them about the kind of products you sell.