I worked with it for a few months, a few years ago. My working memory improved radically, but I was a special case; I was recovering from significant brain damage, so my working memory was already improving radically. Dual n-back was just one of a number of recovery exercises I engaged in. More effective than some.
Can't really speak to its reliability for a general audience, though.
I've run across a couple of questions about the value of dual n-back, and this is the best place I know of to find people who've worked with it.
If you've tried it, has it noticeably improved your working memory? Was it enough to be worth the time?