*********SPOILERS ABOUND BELOW******
*****VENTURE ONWARD AT YOUR OWN PERIL*******
I finished DT7 within days of its release, and had no real problems with it or the series as a whole.
Yes, Martin was vanquished quite easily by Baby Mordred. At the time this happens in the book, I thought there were two reasons for this: the first being that Mordred was one very BAD BABY, the second to show that Flagg was too confident, too cocky, too arrogant, and wasn't nearly the foe he made himself out to be.
Mordred, however, did not turn out to be so BAD, and I think this falls into the same category as his father, the Crimson King. Rarely does the boogie man we all fear turn out to be half as bad as our imaginations carve them out to be. Not knowing what lies beneath the sheet is always more terrifying than turning the sheet back to find nothing but a lumpy pillow...I think that King was trying to emphasize this with both The Crismson King and Mordred, and in all fairness, Mordred actually contributed to his own weakened state by greedily eating the Dandelo's poisoned horse meat, and the Crimson King was completely insane by the time we finally get to see the M.F.
As for King himself being a charachter in the book...BFD! King has always drawn references from the real world (as we all know it) into his fiction. That he should bring in a charachterization of himself was no different than pulling in the Wizard of Oz, Harry Potter, or a half million other things into his work. Its there to pull readers into the fantasy and think that, maybe, just maybe, all this is really happening, albeit on another level of the tower (Do I believe that you ask? Well, no of course not! But do some people? Maybe...Maybe).
I think the novel worked well on several levels, and the ending was fitting, but a little predictable. Eddie's death was a fluke, and shows just how vulnerable humanity is...Life can slip away in the blink of an eye, though Eddie's takes too long in my opionion. And come on, you knew people were going to die. I think Susannah should have been the one to die first, but that's just me.