An agent who is completely new to the publishing field may be very eager to work hard for her clients. A new agent may have more time to devote to clients. On the other hand, she may not have the connections a writer wants. Ultimately, it's a personal decision the writer has to make. Let's say I've signed with an agent, but after a few months, I don't feel good about the relationship.
Most agents have a termination clause in their contracts.
If they do not, a writer should ask for one. Usually, the agent needs time to receive responses from any manuscripts that are out at publishing houses. This, too, depends on the contract. The agent should be aware of what the writer is doing, especially if the agent is shopping the same manuscript. These are the kinds of questions a writer should ask when the agent agrees to represent him.