Flex slider is an image rotator module presenting the advantage of adapting automatically the slides (pictures) size according to the browser window size. You can use the Comparison of Rotator / Slider modules to chose the module that best fits your needs.
Flex slider is not as easy to use as Dynamic display block: Flex slider doesn't provide for a block. Unlike Dynamic display block, Flex slider requires you add a field to the content types you want to add a slide show to, and requires you upload the pictures one by one. Nevertheless, Flex slider provide size adaptation for free, while, as for now (January 2013) one has to pay to get a Dynamic display block theme providing for this functionality. Neither Flex slider nor Dynamic display block provide server-side image adaptation. For this, you can use Views slideshow + Responsive images and styles, as explained there.
The official documentation of Flex slider is a bit cryptic. There are 3 ways to use Flex slider: alone by fields, with the module "Views" or with the module "Views slideshow". We describe here the 1st method.