They were quite good; though I think the first book stood out the most. he did a nice job taking all 4 books to really tie together all the themes. Normally I view books with a TimeTravel element to be a sign of shitty writing (author wrote himself into a corner), but the way it was done in these books was integral to the theme, without dominanting it, and no obvious plot rescues required it.
If you do books-on-tape (i.e. are stuck in your vehicle a lot, so "read" you books that way), the very first book was truly excellent, because they brought in voice actors for each major character - very very good ones. I would never have been able to read in my head the Poet nearly as well or as interesting, as voice actor was able to pull off that roll - it was remarkably well done.