I don't have any issues with the chemistry. I think they are perfectly fine. Without spoiling there are some scenes where Kang Joon is staring at Seung Yeon and you can really feel the chemistry. Gosh it even feels like his glaring right through my soul. Those who say there isn't most likely because they don't like either the actress or the character. From some reviews I've come across I just laugh because they must be watching a completely different drama to me. 

They have good chemistry. I like how they're more like buddies and pals, that makes their interactions more down-to-earth and relaxing to watch. For me, a good couple is when two people can truly be themselves and be able to laugh with, have fun with, and comfort each other. The joking around, sarcastic remarks, and body contact show that they are really comfortable around each other like lifelong friends.

Their chemistry is so good like what are those people smoking? 

While I surprisingly love this drama, it's not their "chemistry" that is a problem, they're fun together, rather it's the fact that the male lead's acting skills outshines the female lead's. Seo Kang Joon is so much more impressive (I think he emotes better). It's unfortunate but maybe it's the way her role was written too. Nevertheless both are talented and I am pleasantly surprised by this drama.