Self publishing is just another option. It's only good or bad depending on the context.
Some good reasons to self publish:
You have a niche that you can uniquely access. (One thing the big publishers are not good at is accessing narrow market niches.)
You believe in your book, and are willing to risk the money, the time, and the effort to give it a chance. (Several successful authors have done this.)
You are doing a limited run for a small group, such as your family.
Some bad reasons to self publish:
Because the big publishers won't take you. (It might be that your book is not ready for publication yet, and by refusing to publish it, the publishers are doing you a favor. Instead of risking your money, risk your time and work on your writing. Improve the book.)
To justify your existence. (I've met some people who really don't think their life is going to have been justified if they don't have their name on a book.)
So you can be known as a published author. (Most people in the book industry look with skepticism on self published books. It's not there aren't good ones, there are. It's just that with the lack of screening and editing, the percentage of good books is much lower. Even if your book is great, you're going to be fighting that stigma every step of the way.)
Because you think there's something magical about having your name on a book. (Having your name on a book means nothing. It's what's in the book that matters.)
There are people who have successfully self published--Christop her Paolini (his parents had a press, which I consider self-publishing, if you publish your own kid), Richard Paul Evans, Hank the Cowdog, , , 50 Ways to Save the Planet. Many successful publishing companies began as self-publishing operations. for example. So there are plenty of examples of people who have pulled it off.
But to do so, you need to do a few things.
Take it seriously. You're setting yourself up as a publisher, not just an author. Don't trust that your printer is going to distribute and push your books. They've already made their money off of you. They don't have much motivation to do any more.
Be willing to risk the money. It's going to cost a lot. Unless you do print on demand. Then your upfront costs are lower, but your per book costs are very high.
Make sure the quality of your book is as good as the national market books. Self published books have a reputation for being poor quality productions.
And it is true that you should not go to a . Vanity presses are called that because they appeal to your vanity. They put on the hard sell, making suggestions that they can do wonders for your books, because your book is wonderful. (To them, everybody's book is wonderful, because it brings them in money.) If you're going to self publish, shop around. Find a press that does a great job at a fair price. (Some self publishers have their books printed at the same Chinese printing companies that the big publishers use.) Get your book professionally designed.
In summary, there is nothing intrinsically good or bad about traditional publishing, print on demand, self-publishing, e-books, any other type of publishing, except maybe vanity publishing. They all have their pros and cons, and if you go into it with your eyes open, and make the right choices, you'll be just fine.