Tonight my girlfriend really wanted to watch The Dark Tower for some reason, and I acquiesced. Everything I know about the Dark Tower series I know from this thread. That is to say, I don't know much. I think I know even less now having watched the movie.
Within the first 15 minutes I felt like I had watched an entire book worth of events and had little idea what was going on. When the man in black mentions in the middle of a big action scene that Roland's guns were forged from Excalibur I laughed at how casual it was, and it was never mentioned again. That line felt like an entire book worth of backstory skipped over with one simple unnecessary line. In fact, the whole movie could have just been called "The Man In Black" because I don't think the dark tower really had much to do with anything in any meaningful way, especially by the end of the movie.
Dark Tower fans, I am truly sorry.