Personally, I thought this show was ok.
As a big fan of Kim Yoo Jung (the only reason I watched this drama in the first place), I didn't really like it. But overall, with my bias and the sweet moments between the main leads, I'd recommend watching it if you're bored.
I liked the cute moments in this drama and their kiss/dating scenes were probably the cutest scenes I've ever seen in k-drama history. Their chemistry in those moments were the best and you probably wouldn't care much about the age gap if you just focus on those. The puppy love between the two main leads are just too adorable so I'm going to commend them for that. I think it's because of the acting between KYJ and YKS but whatever was the reason, these scenes were the best things that graced this show.
Another great thing about this drama was the humor. I loved the scenes of Oh Sol with her family, especially when they're bickering or teasing each other. It was funny and it really showed how weird they were. The side characters were also very fun and interesting so you shouldn't get bored with the comedy if that's what you're looking for.
I didn't really like the main characters though – by themselves at least. The female lead was really naive (in the idiotic way) to the point where I just rolled my eyes at her antics sometimes. She wasn't that bad for a lead (there's been a lot of naive, idiotic leads before) but they should've at least given her a bit more common sense in the beginning of the drama so she could handle the following events better. It would've made her more likeable (at least to me) and she would still have room for character development. Then there was the male lead who was narcissistic and egotistical in the first parts of the show. He was really stingy in the non-charismatic way so I disliked him more than I did Gil Oh Sol. Was there character development? Yes. I think there was. But it didn't really change the fact that I couldn't root for Gil Oh Sol in the beginning and I couldn't find myself liking Jang Sun Gyeol for her either. They ended up creating good chemistry in the end when he stops being an asshole and she becomes a smarter person overall (as was portrayed in the cute moments) but the with the amount of self-importance he had for himself and her knack for clumsily destroying opportunities, individually, I didn't understand how they grew to like each other in the first place.
I personally didn't like the story as much as well. If it hadn't been for the cuteness of the main leads, I would've dropped this show by the time I got to the middle of it. As a lot of people commented, the story gets a bit draggy in the middle and it just became more boring (at least, to me). Still however, the best Gil Oh Sol and Jang Sun Gyeol moments were in the middle so even thought it gets sort of draggy story wise, the main leads became a bit more likeable in the second half too. That's probably what saved it for me.
The OSTs were sort of good but they're not my favorite. Don't take this to heart though, a lot of people have been complimenting the song choices of this drama. You do you since I'm not exactly into those types of songs in anyway.
All-in-all I'd say that this show is ok and it's not the best but if you're looking for something casual to watch, check it out.