I also tacked this image up in another Dark Tower thread, but it seemed appropriate here as well.
This gives us an idea of how their interpretation of Roland will look. To me it looks like they will interpret the story however they want and it will be very loosely based upon the original material. No way they can shoe horn all the relevant content from all the books into one flick. The liberties they've taken with many of the characters and story elements have caused some controversy and outrage among fans. I'm willing to give it a chance (I like Idris Elba), but I'm not getting my hopes up in terms of the integrity of the story.
For more on the topic, you can see this thread:
http://www.sffworld.com/forum/threads/having-trouble-with-the-dark-tower.49651/page-2#post-789033
It started out as a Wizard and Glass discussion, but the conversation drifted a little when I mentioned the incoming film.