Since opening this thread, all I can think of are series where the quality is wildly uneven, so rather than hijack, I started a rival thread.
Two book series that jump right out at me:
Stephen King’s The Dark Tower series. Books 1-4, amazing. Book 7, good and a satisfying end. Books 5 and 6, not so much.
Frank Herbert’s Dune series. Books 1-3, amazing. Book 4, readable. Books 5 and 6, not readable.
Any others?
Star Trek, the movies.
Sure they had an even level of quality; the even ones had quality and the odd ones didn’t. 
That’s what I was going to mention, although if I had to graph the books with Quality on the y-axis, it’d look like this:
1 4
2 3
5 6 7
First that came to my mind is Philip Jose Farmer’s Riverworld books.
To Your Scattered Bodies Go was pitch-perfect, easily a masterwork of science fiction, and even the title alone gives me chills.
The Fabulous Riverboat I liked almost as much, both for the technical details of the hows and whys of building the riverboat and the battle of wills between Sam Clemens and King John.
Each succeeding book was worst than the last, becoming more dissolute and the mythology more muddled. Finally we got to Gods of Riverworld, which is frankly one of the worst books I’ve ever stirred myself to read.
I’d add Piers Anthony’s Xanth books, although their level of quality hasn’t been “uneven” so much as it’s been a slope. The first four (through Centaur Aisle) I loved, but they’ve gone almost uniformly downhill since then. Crewel Lye was a one-shot return to form, but by the time I read my last one (Man From Mundania, I think) I didn’t imagine how it could get any worse.
The James Bond movies. Dr. No and (last year’s) Casino Royale? Terrific! Moonraker and View To A Kill? Terrible!
Kinky Friedman’s mysteries and Andrew Vachss’s Burke mysteries. The idea of a bunch of crazy characters solving crimes in New York City wears thin after a few books.
The Anita Blake books. They started out as interesting horror/detective novels, then took an astounding plunge in quality when the author decided she’d rather write really boring softcore porn.
I guess I’ll be the firs to say Star Wars.
My ratings for each film in the series (5 point scale)
IV: 4
V: 5
VI: 3.5
I: 2
II:3
III:4
Wow, you think Revenge of the Sith and Star Wars are of roughly equal quality?
Yes, I do. While overall, the acting was superior in Star Wars, it really wasn’t all that good…I think nostalgia colors that a bit. I think the story in SITH was more compelling, with better action, and quite good flow. Not that it didn’t have problems, but I quite enjoyed it.
The His Dark Materials books. I adored the first one, moderately enjoyed the second, and barely made it through the third. When people ask me about the series, I recommend reading The Golden Compass and just imagining the rest of the story on your own.
Also, the Back to the Future movies. The first and third are fun, fairly self-contained adventures that explore one time period in depth. I can return to them any time. The second movie seems to get weaker every time I watch it. It’s overstuffed with plot, zips disorientingly between time periods, and is just generally manic and not a lot of fun.
God, yes. I stopped at page, like, 10 of Narcissus in Chains. I had had more than enough.
Long series often do that, though - the Stephanie Plum books started out as pretty smart mysteries, and now they’re pretty paint by number - Grandma shoots something, Lula wears something inappropriate, Sally Sweet shows up in a thong or something, Stephanie doesn’t sleep with Ranger, etc. It’s a shame, because I can’t stop reading the stupid things. (I haven’t read 13 yet, although I was actually pretty pleasantly surprised by 12 - it was kind of a return to form.)
I saw the title and came in specifically to mention Dark Tower. I just got the 1 - 4 set and reread them in anticipation of the 5-7 (I have never finished the series). Now you frighten me!
I was just commenting last night that for some reason it takes me forever to slog through The Wastelands but can go through Wizard and Glass in a sitting. It’s not like **Wastelands ** is a hard read, just not as good so my mind wanders more.
Well, I start on Wolves of Calla this weekend.
Apparently the latest one is better than the ones between it and Obsidian Butterfly (though Anita’s Magical Vagina[sup]tm[/sup] is still going strong).
But another vote for it here.
Don’t be frightened; the series doesn’t jump the shark or get stupid or anything, and I certainly don’t regret reading any of the books. Books 5 and 6 just aren’t quite as brilliantly written - they feel as if they needed more editorial tightening up.
Come back when you finish and let’s open up a new discussion thread. I only finished book 7 two months ago (and only started reading them this year) and really haven’t discussed them with anyone.
And now I’ll stop hijacking my own thread.
I feel a little bad saying this since it’s actually known why the quality dropped off, but The Gormenghast Trilogy. Books 1 & 2 are incredible and then comes Titus Alone.
Damn shame that Peake died at all, but particularly terrible that he died from a disease that destroyed his writing ability before it killed him.
Two Words: Hannibal Lechter
The uplift series was not very even. Im trying to think of more, but nothing is forthcoming right now.
Fry
I was thinking this. Book 1 – terrific. Book 2 – crap. Book three – readable potboiler. The rest stunk.
I normally kinda sneer at “how can I be the first one to mention X” posts… I mean, SOMEONE has to be the first one to mention X. But, my goodness gracious, I would have thought post #3 in this thread, at the latest, would mention Robert Jordan and The Wheel of Time.
The first 5 books were utterly fantastic. Book 6 was a fair amount worse, but with some good parts. Books 7 and 8 were terrible, tedious and plodding with absolutely nothing happening. Book 9 was somewhat better, about as good as book 6. Book 10 was the worst yet. And book 11 was, surprisingly, quite good, almost as good as books 1-5.