This is by far the best free theme out there. There are numerous options for customization right in the theme options. Other themes force you to create a child theme and make changes to CSS to change colors, etc., but Mantra includes most of those options right in their admin screen.
The theme IS responsive. I've tested it on my android and it shows up great.
This theme would easily sell for $50 - $100 on any "theme store" site, so this is a huge gift from Cryout Creations to the wordpress community. This theme actually has more options and features than many of the so-called "premium" themes out there.