In case you aren't familiar with the excellent Writer Beware blog, you may want to check it out.
The post of Feb. 16 contains a red flag alert about:
The Loiacono Literary Agency, which apparently has placed most of its authors' works with small publishers for which you don't actually need an agent to submit, or with vanity publishers;
The Swetky Literary Agency, which often suggests that author needs an (expensive) editor--who happens to be the husband or partner of the woman who runs the agency, a relationship they don't mention;
The Warner Literary Group, which has averaged only two deals per year since it was founded, and has been the target of several lawsuits by unhappy clients. They also have a contract with a clause that gives them the right to represent the work not only for the life of the author but also for 70 years afterward.
You can read the more detailed warnings here.
The ease of self-publishing has shaken out some of the more dubious vanity presses and agents, but it still pays to check out any agent or publisher who offers you a contract and go over the contract with a fine-tooth comb.