Once you have some experience then you will need to decide if the 25 - 30% that an agent takes from your earnings is worth it. Can an agent bring in enough more work, or negotiate better pay to justify it? You are the only one that can make that decision for yourself. Even if you have an agent you should still educate yourself on copyright, contract negotiations, and current trends.
In the current economy some agents have added more illustrators to their stables in an effort to widen their marketing, while other agents have cut back the number of artists that they represent. Study the group of illustrators that are represented to see if your work fits in with the general feel of the agent and yet is unique enough that you aren't competing directly with their other artists. Contact artists who are represented by the agency to see how the experience has worked for them.
If you are accepted by the agency make sure you understand the terms that you will be working under. Will they get a cut of your pay if you find new clients on your own? What about clients you have worked for in the past? What costs are associated with working with this particular agency? You should not have to pay an agent up front but you may have to cover some or all advertising costs associated with their mailings, web page or illustration directory pages.