A lot of writers ask whether it's useful to get an agent. The final answer has to be up to you, but there is one basic question: Do you have the time to shop your book around, or is it more important to you to use what free time you have in writing? This is especially important for those who have a "day job" in addition to their fledgling writing careers. You can't spend all your precious free time researching the right target publishers and sending out submissions ... and still get your writing done.
Agents, at least in the romance genre in the U.S., generally take a 15% commission on domestic sales, and 20% on foreign or subsidiary rights. You should never pay an upfront fee to an agent. They receive their payment only after you have made a sale. Usually, the agent receives the check directly from the publishers, takes his/her cut, and passes the remainder on to you over a time frame specified in your contract. Sometimes, an agent will charge for copying, postage, and other office costs (also outlined in the contract), but should never charge a reading fee.
Agents know the market. They are more likely to know the best home for your book, and will usually have a good relationship with the editors at many publishing houses. A good agent will be consistent in his/her efforts to market your book, and keep you updated on its progress. An agent can be (and ideally, should be) your biggest cheerleader - as excited about your work as you are.
You can submit directly to editors, and sell without an agent, but an agent will help your work bypass the slush pile and get it in front of an editor sooner. An agent can also act as your coach, advising you on aspects of your work that may need to be changed or improved to make the work more saleable. He or she can help you plan your career and make the most of your talents.
Agents do this and more. That's a lot of work, and you might be glad to have an extra person on your writing team to handle it. But the writing part is up to you, so be sure you still have time for it even if you go agent-free. For more information, including a list of questions to ask prospective agents, see the Association of Authors' Representatives, Inc.