Two weeks ago, I talked about some of the benefits of having an agent. Again, I don't have one personally--finding one is one of my goals for 2013--so I'm going by what I've been told by other authors who do have agents.
If you want to go the route of querying and signing with an agent, there are definitely good points to doing so, not least of which is the probability of earning larger advances and being published by bigger houses. However, just as with publishers, there are also things to beware of as you seek agent representation.
First, and most importantly, agents should not ask for money up front. Agents are paid a percentage of what their authors earn, which is another benefit since they have a vested interest in you earning money so they can get paid. Agents do not get "reading fees" or ask for payment for any editing they do for their authors. If you encounter an agent that asks for payment before you've even signed with them or immediately after signing, you may be dealing with a scam. At the very least, it's a safe bet that agent is not a member of the Association of Authors' Representatives, whose canon of ethics includes this statement: "...members may not charge clients or potential clients for reading and evaluating literary works and may not benefit, directly or indirectly, from the charging for such services by any other person or entity." (From the AAR website; see the complete code of ethics here.)
If an agent says, "I know an editor who will spruce up that manuscript for you; she charges five dollars a page. Here's her contact info", you're looking at a conflict of interest. The agent may be profiting from referring clients to the editor. I don't know whether it's common practice for an agent to tell a client to hire a professional editor, but if it is, I would question why. (If anyone has more information on this, please comment; I wasn't able to find anything.)
Agents obviously have more than one client, so it may take a little time for them to respond to a client's questions or concerns. But if they take longer than seems reasonable, or if they outright ignore communication, it should be a red flag for you.
Many agents are members of the Association of Authors' Representatives. If you're seeking agent representation, that organization is a good place to start to ensure that you're dealing with someone reputable. They have a complete list of member agents on their website, along with the aforementioned code of ethics and a page of FAQs designed to help authors. Visit their website here. And good luck in your search!