Being very popular in the mainstream and among "casual" readers is a clear indicator for me that said book series should be avoided.
I hate Harry Potter, I hate the Dan Brown Books, thus it's not likely that i could like Twilight.
Interestingly, the books that seem unappealing at first seem to become my personal favorites. Like "Perdido Street Station". I don't care about imaginative creature-designs, and the "almost sex scene" in the first chapter between a human and a woman/insect hybrid that does sculptures with her spit for a living was a mayor turn-off for me.
Yet, after reading a bit further, i really like the story, unconventional yet very engaging and touching in a certain way.
To a lesser degree, it was the same with the Hyperion Cantos, my two all time favorite books. From the Descriptions it seems to be a very generic Fantasy/Sci-Fi Thing, but it is so much more...