First off, I do not have a literary agent at the present time and have no plans to pursue one for now. I do have several published author friends who use agents and have had decent success with them. I believe using an agent is an author’s preferential decision and often is based on the market you are writing within. Understand your market and the publishers you would like to work with well and then make your decision about whether or not to look for an agent.
There are many purposes for using an agent, including an author’s inability to make the right publishing contacts himself, uncertainty about understanding the contents of a contract, needing a buffer between yourself and the publisher, and knowing that certain markets ONLY look at agented work.
As always, I have added the article on How to Choose a Literary Agent to the Writing Tips on this blog (under My Writing Tips: Finished Product) and to the Writing Tools on my website.