Google Site Search is a quick and easy way to provide a search feature for your Web visitors. I use it on this site but I tend to opt for the built in search feature.
Why? Well, the built in feature tends to look more part of the site. It is fully customizable, and much more professional then using Google's solution. Is it as good? I think sometimes its better, it depends on how I am searching.
A thread at WebmasterWorld discusses this topic. Clint offers some good advice. (1) "If you're in sales of a product, that putting a G search box (or any search box for that matter) that has a "search the web" feature, that this is going to potentially push your potential customers elsewhere." (2) "Atomz's paid version is great", if you can afford it.