Ehh. It's not that you randomly wake up one day and go "Hm, I don't think I like this anymore."
And, note -- I still like the first two, and tolerate the third. It was the fourth that killed it for me. A combination, I think, of the fact that the fourth and fifth sucked, and hanging out on MX, around all the antis, they point out the flaws in the books and you start to see it from a different perspective. It's not really either, it's not like you go anti just because all your friends did, or that you randomly stop liking the books. It's just everything together.
Sorry if that was long. >.< I just wanted to answer your question enough.
I can see that happening (It did happen to me, come to think of it. Which is why I'm not a TwiSquee anymore). It's more complicated and real than I made it look.
. . . . within two and a half hours I went from 'These books are so good' to 'O.O What the hell, I'm not reading that again.' when the third book came out. *kinda did just wake up and change mind on the series*
I always liked them for the funny. I was never looking for plot. Of course, I got irritated at the various ways to say "Flying rocks but being a mutant sucks," but I was able to ignore that and look for the good stuff.