I am not a fan of agentquery.com, as I found out I was listed and I had not asked to be listed there. I created my own page on publishersmarketplace.com, though I don't feel that's the best site for authors, since it's really for the trade. Though if an agent has built a real sub-site there, sure, check it out. I have my own site and list my submission requirements there.
Preditors is a different and more complex story. I think their intent is good, but worry that it's a bit too grassroots. Agents have been blacklisted there without being contacted for their side of the story. And judgment is passed in a manner that fails to make it clear that it's personal opinion. I've gotten "not recommended" there because the moderator doesn't like an optional service—and the related fee—that my firm offers some authors. I may hate the way UPS or FedEx does business, but I wouldn't label them "not recommended" because of it. Companies have the right to do business any legal way. You have the right not to agree with their policy, but think carefully before deciding that policy is a deal-breaker, especially if that firm is offering you something no one else is, such as representation.
My advice would be to look at all of the free resources you can find online and consider the sources carefully. Then make your decision. But if you know the firm reps real authors and sells books, I'd be careful of ruling them out because of something you read on one site or on a forum.
Z