I would have been more satisfied without reading Coda - I was prepared to say, yes, the journey is the most important, Roland got there, excellent. Coda just frustrtated me though - if we know he's looping, and we know he's making a difference (having the horn, smelling roses) then why not show what happens when he finally learns? Or is that secret - when Roland finally achieves redemption, he's not even going to bother opening the door to the tower. (Just read some of Erf's comments and yes, it seems that not going into the tower at all may be the secret,p but why then is the whole centre of existence a shrine to Roland? What's the point of it?)
Ka's a wheel though I suppose, and a mighty heavy one to turn at that.
If the quest is so much about Roland, why did Susannah turn back though? She had a vibe that she wasn't supposed to be there - Roland had nothing to do with that. He let her leave, but didn't start her on that path.
As far as the ending with Eddie/Susannah/Jake, I felt it cheapened their sacrifice. The Jake and Eddie in that New York aren't the ones who gave up so much - if Eddie hadn't lived with his brother all those years, went through his addiction, wuold he really be Eddie? No, he's just some guy who looks like him. Same with Jake. And that disappointed me. To be honest, I would have much rathered see them united at the clearing at the end of the path, even if it meant losing the last member of the ka-tet.