I did about 6 weeks of it. I started with 15 minutes every day, and got tired of working hard, so ended up doing 5 minutes, and eventually, losing motivation.
I didn't notice any effect other than improvement at the task at hand.
I've been thinking about either training some more, or using it as an every-week measurement.
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?