I've done the brick and mortar self-publishing, for non-fiction books. It worked out pretty well for me, and I made a few thousand dollars. It also took me almost a year to get my book out, from when I started writing it.
So I understand everything about the process, from editing, cover design, and layout to marketing, advertising and distribution. It's a lot of work.
This video from Mark Coker over at Smashwords covers a lot of the reasons I went with Smashwords. For about 10% of the profits, they get the books into all the distributors, handle all the ebook conversions, give me a platform, and have some great resources to help you get going.
I'll admit, I considered a few of the more specific online publishers, and even found an agent for one of my unpublished books (which I'm still working with her on.) But for now, Smashwords seems like a great solution to me.