This book is certainly a ride. The story sets things up by slapping you in the face with harsh back-story. The main characters are definitely carrying some baggage around and it makes them a little more interesting as the book progresses.
It's nice when horror offers not only good and evil, but also several shades between. The Nightmare Within offers you characters you can admire as well as characters you flinch from while not completely abandoning your sympathies.
Of course, when he wants to introduce evil into the story, the author has no trouble doing that either with the main villain being suitably evil. The only issue I had with the "nightmare" of the book was that a little too much of the story was told from its point of view. While its intentions were unknown and as its tormenting grew, it remained very creepy, but some of that effect was lost a little when its thought processes and needs/desires were revealed. I think it might have been a more chilling story if the other characters were left with the responsibility of determining and revealing its motives.
This is a story that packs a punch at the start, then builds an atmosphere of unease and creepiness right up until the point that all hell breaks loose. From there it's compelling reading right up to the rather "impactful" finale.
The premise was preposterous, but once disbelief is suspended, the internal logic of the story doesn't hold up too badly.
And lastly, I loved the artwork on the cover. It really is impressive.