The story had so much promise. Great cast, great chemistry, interesting plot points:
1) Who is DoDoSolSolLaLaSol?
2) fated romance of Rara and Jun
3) growing friendships in Eunpo
4) relationship lines of the secondary couples

But then suddenly at episode 14, the writers decided that things were looking too prematurely good for the leads that they had to go through one of those stupid draggy kdrama tropes: a lead character gets an incurable disease and the characters don't communicate, a lead character suddenly gets into a car crash and loses their memory, an ex-girlfriend appears and makes the female lead jealous and the leads don't communicate... you know, ONE OF THOSE.
And the way the writers attempted to resolve this conflict was even worse. Details in episode 16, I've marked this review spoilerless so ask in the comments if you are really desperate.

Long story short, it was such a wasted potential because that big fate reveal was so touching and the main romance line was totally shippable
until the writers decided to just play with our feelings and execute the most unrealistic finale just so they could tell the audience they gave us a "happy end."

I'm screaming. I had so much hope for Lee Jae-wook's big first male lead role, and for the amazing cast of support characters like the doctor and Rara's landlady. Never watched Go Ara's works before but she played this clueless happy-go-lucky girl pretty well. And the mysteries surrounding the leads' past and the identity of the titular character were nicely done. But then... sigh.

Verdict? If you're okay with leaving things open-ended, watch until the airport scene in episode 15.
If you're the viewer that needs closure, forget this drama existed and watch something else (Search:WWW is an amazing rom com where Lee Jae-wook gets a hilarious and cute love line). Do Do Sol Sol La La Sol's Episode 16 will have you screaming in anger.