Firstly, a note about the title: everything on the Internet disagrees if this book is First Among Sequels or Thursday Next. The Library of Congress information on the title page attempts to cut the Gordian knot with the mouthful Thursday Next in First Among Sequels, so that's the title I'll use until I hear otherwise from Jasper Fforde.
The Eyre Affair was rightfully celebrated for its intricate cleverness. Five books later, what was once "intricate cleverness" now is indistinguishable from "boorish obtuseness."
I've lived my life since I read the last book, the title of which currently escapes me (Something Rotten?). If I don't remember the big things, don't write your book assuming I remember the minutia.
The ChronoGuard plotline has swallowed itself. Imagine the end of "Back to the Future III" going back in time and meeting the beginning of "Back to the Future." That's the kind of time enigma we're dealing with here. The wormhole offers decreasing returns to intricateness, and I maintain its first derivative now equals zero.
Thursday's old now, and has to interact with a caricature of herself, which is equal parts funny and annoying. Funny because of the twist on the reader's perception of Thursday from the earlier books; annoying because three main characters are named Thursday, which makes the reading more confusing.
Fforde has more books planned. Without any over-arching plot I'm invested in seeing resolved, I just lack any excitement to read on.
Rating: four and a half out of seven giant inflatable monkeys.