Well, the first 4 books are some of his best writing, I think... The final 3 suck terribly, and were a great disappointment (to me, of course, maybe some people actually liked them)
Overall I agree, but I actually quite like the ending of the series. I've read lots of folks online who absolutely hated what ultimately happens to Roland - I thought it was a cool ending which was very much in keeping with the sort of "cosmic cycles" theme of the books. That said, there is a _lot_ of junk in those final three books, and the seventh one in particular drops the ball in several disappointing ways before it ends.