I'm not a very organized reviewer, but here goes nothing.

Misaeng was a really good drama. I'm impressed that they were able to keep 20 episodes of office-work interesting enough to keep me hooked. While I think Misaeng is a really good drama and a must watch, the reason I don't think it will suit everyone is for this exact reason: 90% of the drama happens inside of an office.
I appreciate many kind of dramas but I love "serious" dramas the most. If you're the type of person who highly prefers romantic dramas, I doubt you will like Misaeng.

In my personal opinion, the acting in this drama was its strong point. While everyone did an alright job, two actors blew me away: Lee Sung Min as the kind hearted and hardworking Oh Sang Shik, and Byun Yo Han, who's character Han Suk Yool who evolves from sleazy pervert to a reliable friend who looks out for everyone. If not for these two, I think the drama would be an 8/10 or worse. The lead actor, Im Shi Wan, has the whole story revolving around him but doesn't do anything with it. I don't have any experience with him from other dramas so I don't know if he is at fault or if it is just the role that made him look that way, but he besides being handsome he didn't do a whole lot to be memorable.

I want to finalize with a couple of key points that I want to point out:
-The character development in this drama is really good. Every main character evolves in a clear yet subtle way, which is somewhat atypical for a Korean drama.
-Some episodes don't end with a cliffhanger, which was actually really refreshing and nice. It was easy to lay the drama down but the next day you would still find yourself wanting to watch the next episode.
-There are many foreign languages being spoken by many different actors as they have buyers from all over the world. All of these were incredibly cringy, especially English and German sounded awful (Only Chinese sounded somewhat natural). I know this is a problem many k-dramas have, but I like to point this out as it is a pet peeve of mine.
-If you happen to love Kang Ha Neul (the last drama I watched was When The Camellia Blooms so he grabbed my attention) and want to watch this drama for him: He does a good job but his character is quite boring, don't expect too much from him.

tl;dr: I'd recommend this drama to everyone who thinks he would enjoy watching 20 hours of Korean office dynamics. If you want romance or action, this drama is not for you. If you like realistic Korean office politics and good character development you should 100% check Misaeng out.