I've read all of the Thursday Next books and the two Nursery series books.
My assessment is that Fforde is a second-rate imitator of far superior authors. The Thursday Next series is his extended takeoff from Woody Allen's classic short story The Kugelmas Episode. Fforde is clearly influenced by Pterry and Doug Adams and very painstakingly tries to outclever them. In spite of his derivative style, The Eyre Affair was a pretty decent read. Every additional book in the series has seen a geometrical drop in quality, humor, originality and sense, until the last two were nearly unreadable.
The first Nursery book--which apparently Fforde wrote before The Eyre Affair and then rewrote later--is, for me, his best book. The second book in that series is just plain awful.
There's far less to Fforde than meets the eye.