The AAR list is important when you're checking up on an agent's legitimacy--not all successful agents are members, but membership does suggest that the agent is reputable. However, it's not a good place to look for agents to query--there's just not enough info about specialties and interests. I'd say that AgentQuery is a better resource, because it provides quite a bit of detail about those things, and in addition mentions sales.
One caveat: always do some additional research. AgentQuery is the best of the online agent listing sites, in my opinion--but like any listing of agents, online or off, there are some agents who aren't as desirable as others. Also, despite the best efforts of the site owners, information can be incomplete or out of date. If the agent/agency has a website, that should be your primary information source.
- Victoria