In my experience they are a total waste of time.
When my DH was looking for a new position 7 years ago he submitted his resume several time to these search firms for jobs that were actually posted on their websites as being "open". Not one call, ever. Now that he's an executive with his current organization he gets about a call a month from recruiters.
Generally speaking these organizations want candidates who are gainfully employed and successful in their current job. People who submit their resumes to these types of "executive search firms" are generally thought of as being unhappy, and/or unemployed and thus not as desirable. I said "generally", not always.