It mainly suffers during the middle books. Jordan seemed to lose his way a bit. It went totally off the rails with Crossroads of Twilight, but kind of pulled it back a bit with Knife of Dreams. I've heard the final 3 by Sanderson pulled it all together fairly successfully.
The final three were my favorites in the series.
My general impressions of the series:
Books 1 through 3 were very good reads, though Jordan's wordiness already showed at times.
Books 4 through 6 were still good, but the plot was beginning to slow down.
Books 7 through 9 were a bit of a chore to get through. At that point, though, I was so invested in the characters and the story that I had to keep reading.
Book 10 was bad. No other way to put it. Nothing happened.
Book 11 was an improvement, probably on par with 4 through 6 for me.
Books 12 and 13 were very good. For me, they were better than the first three. Sanderson might get wordy at times, but he's nowhere near as wordy as Robert Jordan.
Book 14 was one of the best I've ever read. It was pretty much nonstop action and one of the most relentless books I've ever read. Those 900 pages went very quickly.
