Judging from the title, I expect this drama will be like Rich Man Poor Woman J-Drama, which depicts the story of actual Start-Up life and the conflict people within it. Which turns out.. Wrong.. Pilot episode is different from what I expect..

Rather than focusing to the foundation of business motivation in the Pilot Episode, the story focus to build up second male lead and female lead connection, through letter. Moving on episode 2, it's become more clear that this story will focus more on romance rather than actual start up life.
Otherwise, I will write the review from my experience watching it:

- FIRST HALF (ep 1-8)
Although the story consist of 80% drama/romance, 20% actual startup insight, this is quite enjoyable story up to episode 8. As viewers, you will be divided into rooting for Male Lead (DS) or Second Lead (JP), which makes this drama so hype and popular at the time it's been airing. There's nothing wrong rooting for Male Lead or Second Lead, because triangle love has been main focus of this drama, and it's up to your preferences, which character you find more relatable.
Note:
I personally rooting for JP from the start, because I just can't stand the relationship start based on lies, whatever the reason behind.

- SECOND HALF (ep 9-14)
By the time the truth has been reveal to Female Lead (JP who actually wrote the letter, DS only pretend while he never knew Female Lead before),
This is the momentum to turned the table, make viewers rooting for Male Lead who will end up with Female Lead.
But the writing and the story have been failed miserably catching this opportunity.
Rather than make the DS more favorable Male Lead, instead they giving so many red flags to his action:
- Backstory about cheating during math olympiad
- Following and stalk Female Lead while she said she need space and time to think
- Saying the Female Lead is his dream, where there's no specific reason/trigger he fall in love with FL (rather than it's because FL is the first girl interest in him, and she's pretty - shallow, eh)
- Punching his mentor because the mentor stating the facts (as some people point out, DS might have anger management issue)

And for Second Male Lead, I think the writers are not intend to make the Second Male Lead more likeable at first, but the actor (Kim Seon Ho) who played JP doing outstanding job portraying the character. We see his character has been consistent in each episode and giving more impact whatever the story he's in.
Some action who let him win the viewers:
- How selfless he's being, when he intend to get the last letter and giving it to DS so the lie will prevail
- How he explain the lie to Female Lead, with not bringing down Male Lead, and rather he justify DS action (only fair play)
- How he confess in calmly yet professional manner
- The viewers know his backstory from episode 1, his relationship with the Grandma, and how lonely his life so far, which makes so many viewers rooting for his happiness (with or without the Female Lead)

From what I see, majority of the viewers are rooting for Second Lead from the progression and how the story been carried up to 14 episode. And up to episode 14, Female Lead not yet decide with whom she will end up with, which causing debate here and there (again make this drama more hype, which team are you?)

- THE ENDING (ep 15 -16)
This is the challenging part for the writers, how they gonna wrap up this mess because of the ongoing debate, and make it plausible ending?
To make DS end up with FL, they must giving the viewers clear reason in FL POV, but it's hard giving the condition, up to ep 14, almost none character growth for DS.
Or to make JP end up with FL, like majority the viewers want, and make this drama exceptional.

Again they failed miserably in handling the ending, which turned into.. A bummer..
The ending is one of the most absurd ending i've watched
- No clear reason why FL choose DS (rather than that cliche and lame reason)
- No resolution and character growth for DS, yet he get the girl because he is the Main Lead
- Almost no closure from JP and Female Lead relationship, JP even have to justify why Female Lead choose DS in their last scene, which most viewers don't buy it..

To close this review, if you are curious the hype of this drama, the story is nothing much except triangle love which dragging until the very last minute.
There is almost none morale of the story from this drama.

But I'm still glad i found this drama, cause from this drama I knew the amazing actor Kim Seon Ho exist.
I've watched his drama from Chief Kim, but he's not noticeable in there.
Here in Start-Up, his outshine other character, and It's safe to say Han Ji Pyeong will be the most memorable character in K-Drama history.