I liked the first six books, because they were interesting epic fantasy books in the vein of Robert Jordan's The Wheel of Time, but then the quality began to drop fast, because the author concentrated too much on writing and preaching about philosophy. If the preaching doesn't bother you, it's possible that you may enjoy the whole series.
I have to admit that I didn't like the last books at all (it was quite an ordeal to read them).
(By the way, I advice you not to read The Law of Nines. It's a very bad book.)
Terry was killing it for the first 8-9 books. but when he started racing his formula one car full time his writing dropped off a lot. just my opinion. its still a good series just takes a downward turn late in the series