I'm probably the last person who cares about this to have their say , and everyone else has said it better. But say it now I will, rather than waiting ten years:
I'm with Sai King; I'm not keen on the ending, but it's the only one that make sense. Once the story becomes metafiction, and we know that ka is a wheel; when it becomes clear that the Dark Tower is The Dark Tower, then what else can Roland find in the last room?
 If you've not read it you'll probably saying "Seven books? Seven? You expect me to read seven volumes? That first one is really thin - but look at the price! And the last four are really thick - how do you expect me to get through them?"