Personally, I had to be forced to read three books into the Twilight series. Finally, I refused toc ontinue on and convinced my teacher to allow me to read another series as long as my assignments corresponded with the rest of the class's. Vampires that *sparkle*. Need I say more?
As for Harry Potter, I must admit that I feel bittersweet concerning J. K. Rowling's "masterpiece/lifetime achievement." Along with half the world, I began the series when I was particularly young and not quite yet experienced enough to know the difference between good books and anything I thought was interesting and fueled my imagination. However, I can safely say that the great, magnificent ending I always imagined from the books was one of the worst disappointments of my brief life (i.e. trumping Dumbledors and Sirius's deaths).
Ultimately, though, I have to admit that at some point in every series' career I start expecting the "turning point," when the series has gained enough popularity that the author/creator can either loosen their own reigns and write what they want and not pander as much to what they expect fans will read or the integrity of the concept is sacrificed for the sake of material gain (i.e. Fast & Furious franchise). The same goes for all great fandoms: Buffy, Charmed, Anita Blake, etc. I fear it may be inevitable....