No experience with resume/CV-writing services, but if you decide not to use such a service, the guide “Applying for Federal Jobs—Federal Resumes and KSAs” from Duke University’s Terry Sanford Institute of Public Policy is extremely helpful.
It doesn’t appear to be on the Sanford Institute website any more. I plugged the URL into archive.org, but I can’t tell if archive.org actually captured the PDF. Try this link (but it might not work): http://wayback.archive.org/web/20110101000000*/http:%5C/%5C/www.sanford.duke.edu%5C/career%5C/graduate%5C/documents%5C/fed_resumes.pdf .
See also the discussions from the 1/19/12 and 2/7/12 Rant/Revel threads — http://www.princeofpetworth.com/2012/01/random-reader-rant-andor-revel-419/ and http://www.princeofpetworth.com/2012/02/random-reader-rant-andor-revel-432/ .