I believe it was an amazing ending. My only criticism is that it was spoiled by an explanation from King. It seemed like he knew he would get destroyed by some fans and had to defend it.
I will tell you why it is such an amazing ending. The ending is amazing not because it is "good" or "happy" but because it is just. Roland as he is depicted throughout the seven volumes does not deserve a better fate. I think his ending is a commentary on the protaganist and on life in general.