When I first got into them I was addicted, but that was extremely short lived. After a while I just found them painfully cliche, dragged out with forced drama, terribly acted and directed, and full of weird shitty behaviour, like men forcefully grabbing women and women who can't act like they enjoy kissing.
That said I really enjoyed them at the time, many of the things I dislike about dramas worked in their favour at first (like the cliches and drama). They have a lot of good looking actors, nice sets and good production values, and the romance and emotional moments can really get you hooked. I remember liking Secret Garden, Coffee Prince, My Girlfriend is a Gumiho, You're Beautiful, Sungkyunkwan Scandal.