Yelp already has ratings for individual agents. More useful, however, are the insurance ratings published by J.D. Power, based on consumer surveys: http://www.jdpower.com/consumer-ratings/insurance/index.htm.
When I moved from the UK to the US it was rather bizarrely up to individual agents discretion as to taking me on. In general everyone quoted you the same rates as a 16 year old who had never driven before, because you had no insured time in the US. But one agent was prepared to take non-US driving experience into account.