I self-published both of my novels so far (via Authors OnLine, a British firm which offers a "menu" approach). For the first book, there was a reason why I wanted it published by a certain date and, after a few attempts at contacting agents and publishers, I realised that my deadline would never be hit this way.
The first book sold very well by the standards of self-published novels (over 1,500 to date and still selling steadily). Having learned more by then about the difficulties of getting published traditionally, I took the same route for my second book. This has not taken off yet, and I think I know why: the first book was an alt-WW2 story and there is a large group of enthusiasts for this, who are easily reached via the web. The second book is mainstream SF and just gets lost in the ocean of works on sale. Admittedly, I put very little effort into self-promotion, beyond my website and blog.
So the moral is, if you have a niche product for which there is an identifiable and easily-reached market, self-publishing may well be worth it. If not, it probably won't be unless you work your socks off (and maybe not even then).
For those wanting to know more, I have posted a long article on my publishing experiences and opinions here:
ON PUBLISHING FICTION