Attempting to keep this brief.

Kim Hye Yoon and Im Sol are as perfect a pairing of actor and character as can be achieved by human effort.

Byeon Woo Seok is more than capable as a lead and is particularly good when Seon Jae is uncomfortable and awkwardly trying to make something happen with Im Sol. Somehow, when he's supposed to be cool and under control, doesn't seem as natural.

The two together are fantastic particularly through the first time loop sequences up until Im Sol finally (in present day) comes clean about her feelings and the big scene in her apartment doorway delivers the awaited romantic payoff.

These two can duke it out with the lead couple of just about any drama and hold their own.

But the rest?

Not great.

There's a second couple but 1. it's forgotten almost completely from episodes 4 through 15 and 2. they are clearly intended to be the comic relief in a show where the leads provide plenty of laughs so the second couple has to take their acting to a sketch comedy level that is nails-on-chalkboard grating.

Kim Won Hae is great (as he almost always is) but the relationship between Sun Jae and KWH's father is rarely given any in depth play. What could have been a terrific subplot, particularly when dealing with Sun Jae's swimming career, is ignored.

Tae Sung is a promising character but he needed a bit of backstory and consistency. Like a lot of characters that aren't the main two, Tae Sung is too frequently simply a convenient device that transforms in to whatever the story requires him to be. A bad boy that needs to fake a relationship with Im Sol. Later a buddy for Sun Jae. Then an object of jealousy. Intermittently, a character teased as the third spoke of a triangle (thankfully only teased but even the teasing of it was unwelcome). Finally, a cop to pursue the villain. Although the time loop changes can account for some of this, it's jarring when in one loop he's a successful fashion entrepreneur and then suddenly he's a civil servant.

Back to the villain: it's the most bland villain ever and the final confrontation with him is so painfully anticlimactic.

And the storytelling overall is lacking. It starts after major events that drive the storyline have happened but the context is never adequately exposited. The time loops are fairly well handled but there is absolutely one more loop than there needed to be and it's clear that the writers grew tired of incorporating how the most recent loop back changed the present day circumstances.

It's a show that could have 1. been a show exclusively about the lead couple for about an 8 episode run (like a longer "Soundtrack" drama) or 2. a more traditional 16 episode drama that invested more screen time in the secondary stories and characters. Either way, with this lead couple, it could have been an all-time great show. Instead, it's a fun watch and certainly recommended but falls well short of what it should have been.

[EDIT: Wow. Thanks so much to everyone who has liked this review! The positive feedback is very appreciated :)

The review should have mentioned that Im Sol's grandmother, both the character and the portrayal, were wonderful but, frustratingly underutilized.]