I get asked this question many times a week via twitter, in emails, at conferences, at the bookstore... Here's how it goes:
[scene: a podium in a random hotel conference room in Anytown USA]Oh. You can tell I'm poking fun, and you don't like it. Sorry. I'm going to be totally serious now. When you ask that question, it sounds to me like "but I don't want to read the manual, instructions are for suckers, I'm gonna find the shortcut and do this the fast and dirty way!"
Stranger [raises hand]: OK, SO, PEOPLE SAY I SHOULD LOOK FOR AN AGENT. BUT MY QUESTION IS, DO I NEED AN AGENT?
Me: Probably. Depends what you want from your career. I'd get one, personally... but of course, I'm an agent, naturally I'd say that.
Stranger [impatiently]: NO. I MEAN DO I REALLY NEED AN AGENT?
Me: Oh man. Since you asked with that inflection... wow. I guess I have to give it to you straight. You got me. There's a little something that everyone else knows except you. Every time somebody suggests you "query agents" they are really trying to TRICK you. Because they don't want you to know the secret! *Muwhahaha!*
And I get that. I'm impatient too. The fact is, much like putting together the elaborate entertainment center from Ikea, getting an agent is usually difficult, or at least inconvenient on some level. There are almost always some bruised egos and frustrations along the way. It can take a long time. It can be a lot of work. And even when you have it figured out, that is only the beginning. Ugh what a pain in the ass.
So look. It DOES depend on what you want from your career.
* If you want to go the self-publishing route and would never consider traditional publishing... you may not need an agent. (You'll find that it is a lot of work to be successful at this game, but hey -- it happens. Elbow grease, baby!)
* If you have a very "niche" type of work -- highly technical, educational, religious, or specialty-type content... novellas, chapbooks, and other things that are not usually found in regular bookstores... you may not need an agent.
* If you want to be traditionally published and are a super-type A personality, know a ton about the vagaries of publishing, have lots of insider publishing connections, know contracts well, understand the market for your work specifically, enjoy talking about money and don't mind things like asking for a raise, know how to sell subrights and foreign rights, want to spend time pounding the pavement on your own behalf... you may be not need an agent.*
Otherwise? An agent is going to be hugely helpful to you.
In fact, here's the secret. As big a pain as it is? GETTING AN AGENT IS THE SHORTCUT.
You can put the Ikea entertainment center without it, but dear god, it's a hell of a lot easier with it.
Consider an agent your hex key.
* (Though I do know people like this, and after a certain number of books... guess what, they got an agent. Because it is a huge time-suck to do all these things for yourself, the time-suck gets exponentially worse the more books you have out, and that is time you could be spending writing. Or, you know, lounging in a hammock drinking mocktails. Whatever.)