Loved the cast, acting and accompanying music.
Pace not bad, although it flagged a little around half-way.
What really let this drama down, was a poorly thought out ending. There really are gaping holes in it, which totally let down the entire story for me.
The relationship between 2 of the characters was ambiguous and I felt a little uncomfortable.
The character of Jin-gyum was so naive at times, about the cause and effect of time travelling and parallel universes. He really said and did some dumb things. Tae-yi could also be irritating, although she was pretty strong willed at times, whilst at others she's just stand there instead of assisting someone being throttled!
I also felt some characters were just there to fill out the cast, as they really served no other purpose.
It would have been so much better with a more plausible ending and it's not a drama I'd watch again.