I did a fair amount of research in the quest for a two week rental on Cape Cod. After trying the 'friend-of-friend' route, it became clear that an agent was the only way to go. And while the Cape is literally swarming with agents, I found that there is a fair degree of area specificity ('we serve Dennis, Dennisport, and Chatham') and that in each area, there is one top firm that has the largest inventory.
Martha Murray appears to be such a firm, with one of the best Web sites. We wound up working with them and they proved to be responsive and honest ('while that house is more expensive, I don't like it very much so I would go with the cheaper one'). I have recommended them to others and they have had similar experiences.