I couldn't care less whether something is popular or not - if I like the sound of it I'll read it - and if I don't, I won't, it's as simple as that. I read quite a lot of books when they're in the chart - but equally I read stuff that's donkey's years old.
I won't read Twilight because I'm not at all interested in vampires. In fact, I'm sick to death of them - they seem to be everywhere one looks at the moment!
I read Harry Potter and the Philosopher's Stone when it first came out but far from going off it when it became popular, I loved them until the last one and I'm so pleased for JKR and her success.
I never quite understand why people go off bands when they become mainstream. I'm pleased that a group I've enjoyed have had success and (hopefully) made a shed load of money! But that's just me.