Although I find Twenty Twelve easy to use I don't find it very responsive. Every time I click on a new page in my navigation bar blue links very briefly show behind each page which is not good. My site is at http://www.fatlossfactorreviews.co.uk and you can see what I mean when you click on each page link in the nav bar.
Really love this themes,Clean and Fast
I think people considering using Twenty Twelve should be aware of a serious compatibility issue with IE7 and IE8 - they will only display the Mobile menu. IE7 and IE8 still account for 15% of all web browser share, and IE8 will remain for a long time - there are still a lot of XP machines around.
I think Twenty Twelve's incompatibility with IE7 and IE8 is currently a fatal flaw - it really is not okay to have the Mobile menu the only menu displayed on IE7/8. While the percentage of IE7/8 will drop, right now that means over 1 in 10 visitors will see a broken site.
Otherwise, the pure simplicity of Twenty Twelve is quite nice, and it does look like it will serve as a good parent theme - but only after the IE7/8 problem is fixed.