I've found a few ratings sites, but nothing jumped out at me. It seems like it would be a great service that someone could provide, as long as they could avoid being sued by the recruiting agencies that got bad names in the process.
People who hire a lot almost universally have horror stories about bad recruiters. Any friend of yours referring a good recruiter should be able to explain what sets them apart from the vast miasma of boiler room telemarketer rip-off artists.
I would read every review though. Similar to @neworbit's comment about recommending specific recruiters vs. services, I had an great experience with a recruiter (he also helped my friend get a fantastic job), but since he left, the agency he worked for has gotten some really nasty reviews.
I have used a lot of recruiting services in the past. There are nearly none that I would recommend, and only a couple of specific recruiters I like.