I remember when I first got my hands on the first book. I couldn't put it down. When I finished, I immediately went out and bought the second. Upon completing that, I immediately bought the third. When I finished that one, I went to the bookstore and discovered that the fourth had not yet been written. After each passing year, I hated Stephen King more and more because after three books, Roland was still 8,500 miles from his Tower.
The only reason he finished the series is because he got sick and tired of receiving letters from readers reminding him of his promise to write seven books. You could notice a shift with book five as it began to venture from a believable futuristic story into one that defied reason. Reading on, you soon discover that villain Flagg shows up, completely out of character, and then you realize you've been had. I really wish now that I had stopped at five.
I was the same. The hunger for the series continued until the 6th book. By the 7th I was just reading to finish what I started.
So sadly, I already know that King really screwed it up in the 7th book. I bought the 6th and 7th as soon as they were printed, unfortunately.