Today, self-publishing has become a widely accessible option for many writers, and self-publishing your book can be very satisfying, depending on what your goals and expectations are.
You want fame and fortune
Don't we all? If your goals in writing and publishing your book are fame and fortune, know that even most traditionally-published authors don't experience that (for more about that, read the About.com article on what to expect as an author). While there is the occasional blow-out bestseller success story, industry experts claim the average self-published book sells only 150-200 copies, and that's mostly to friends and family.
But there are other, very valid reasons to self-publish.
You want a keepsake or a remembrance
Self-publishing allows you to own a hardcover collection of your personal poetry; to commemorate a family event; or to give your siblings copies of your mom's recipe collection. Print-on-demand self-publishing companies like Lulu.com allow you to print as many--or as few--copies of your books as you'd like.
You're an expert in your subject and have a popular blog/radio show/seminar series
Self-publishing is a great option if you're an expert with a platform--a blog with thousands of subscribers, a regular speaking engagement schedule, a radio show or other large following--and an audience who you know is very interested in your topic. For example, About.com's Patient Empowerment Guide, Trisha Torrey self-published The Health Advocate's Marketing Handbook using Amazon.com's CreateSpace--Trisha knew her About.com readers would be interested the subject.
You (all) want a fund-raiser
Self-published "community" books (like Junior League or church cookbooks) have long been used for fundraisers; collected remembrances of Hurricane Katrina were published in a book to benefit the victims. Self-publishing companies like Blurb have models for charitable publishing.
You're an avid sci-fi/fantasy/mystery/romance (or other genre) buff, you know what your fellow fans love and you've written something they'll enjoy
Many voracious readers of genre fiction are moving online and turning to ebooks, which is how bestselling author Amanda Hocking got her start. If you know your genre and audience really well and know how to reach your reader (because you're scouring the same places), then self-publishing an ebook might be the way to go. Some companies (like Barnes & Noble's PubIt!) offer a distribution component, which could help build your audience of readers.
You've written a children's book / a brilliant literary novel / a memoir about your beloved dog but can't find a publisher, though you know many people would appreciate it.
There might indeed be a waiting audience for your book--but see "average self-published book" sales number, above. If your aspirations are literary, it behooves you to take a step back and consider any constructive feedback you've been given (family or friend raves don't count). Make sure to address any issues with your plot, your characters, your writing--and your typos. Many self-publishing companies, such as iUniverse, offer editorial feedback, for a fee.
Because while the process can seem easy and, from some services, free, if you care about the quality of what you put out into the world and want to reach readers, self-publishing is not typically either of those things. Traditional book publishers set a high bar to entry because they know the publishing process is labor- and resource-intensive. Self-publishing is no less of an investment--and it will be entirely your investment. Make sure the content of your book is as worthy of your time, money and energy as it can be.
Whether your goals are personal or professional, to reach a large audience or to make Mom smile, it makes sense to take stock of them before you self-publish your book. Then, when you hold your book in your hands, you'll be better able to gauge your self-publishing success--on your own terms.