Though the premise is good, the writing of this show is simply overall too ridiculous and make anyone's suspension of disbelief really hard.
However, there's still something good:
1) the cast
2) the fact this k-drama belongs to a new "era", where characters can finally have real physical interactions according to their age, which still surprises me. It's quite refreshing.

To sum up: it's ok as a filler drama, maybe not too much if you're looking for something that will catch your interest more.