Like Adina, I too fell into the trap of the 50 Shades trilogy. [And unlike Adina] I read them all. Y'all, they are bad. Like really bad.
In the case of both of these series I (like many others I presume) felt like I had to read them to see what all of the hype was about and to form my own opinion. These series though really fall deep into guilty pleasures category. Actually... I really just feel guilty about it.
On another note, as someone pointed out, I think what we define as a guilty pleasure varies with each individual. Though he is pretty popular, Nicholas Sparks would easily fall into my guilty pleasures column. It's been several years since I've made it through one of his novels. When I was in high school, I quickly became obsessed with A Walk to Remember-- first the book than the movie, of course. And then there was The Notebook. Later in college I read a few back to back (thank you college roommate), but honestly can't remember which ones they were. They all started to merge together as one story in my head.
These days, I don't find myself reading a whole lot of books I would feel ashamed of. I mostly stick with recommended books or reads I know that will make me laugh or intrigue me. I'm trying more now to go back to some of the classics that I would appreciate more or never had the chance to read.