I haven't try adding a google search on my site. I dont think it would help you on PR/SERP but it would help your visitors to search your site or the web much easier.
I would say that it doesn't aswell and from what I understand is that it can only search through pages that are already indexed in Google.
You'd be better off havinga good internal solution, customised to your URL structure.. But better still have an URL structure that is Google-friendly