So it definitely transfers 'near'. You'll get better at the dual-n-back test. And possibly other visio-spatial working memory tests. But the question of far transfer- will it make you smarter- is probably not. Here is a link to a 2015 meta-analysis that looks at the question of working memory training (including dual-n-backs) transferring to other working memory tasks:
https://www.dropbox.com/s/084fvteji1tyz8t/2015-schwaighofer....
Melby-Lervag & Hulme, 2013, is also pretty damning.