I found that the Nagash books were fine, unless you were expecting them to match up in any way to the existing background, because they just didn't. Moreover, while, despite being a "Lore Guardian" (or Fluff Nazi if you prefer) I'm generally of the opinion that I don't mind changes to the background so long as they're for the better, I felt most/all of the changes in the books were for the worse. There's also a sense that nothing happens in the second book and we get the catch-up plot from that in the third: the third book feels cramped. But it was the background concerns that bothered me.
I hated the Sigmar books wholeheartedly: the first one was barely tolerable; I thought the second one was particularly awful and I found it difficult to form a clear opinion on the third because I'd hated the second so much - I didn't enjoy it though. May there never be a fourth (although the author has threatened one).
I thought the Sundering trilogy was very good, though, aside from the first half or thereabouts of Caledor which felt like a re-hash of stuff from the first two books.
Age of Legend was pretty good, and I thought Dead Winter was excellent.