If you enjoy a fluffy experience with some suspense and drama in between, this is for you.

The story is well-written and thought-through. It's nothing too deep but it was entertaining and suspenseful. It had the perfect balance of 'setting the tone' and 'important plot points'. There were some cringe-worthy things here and there but it wasn't too much and easily ignore-able if needed.

The acting was great! Especially Cha Seowon did an amazing job of portraying Taejun here. I was somewhat worried about Gongchan, considering that I knew him from B1A4 only, but he did well. There is room for improvement, sure, but if he keeps at it, I'm sure he'll do even better in his next project.
It's a Korean Drama alright, so you will have 'those' moments. But in comparison to other BL's, they are acted out well and you can feel the chemistry between the Main Couple. It's absolutely adorable.

The cinematography was amazing. Very beautiful camerawork and editing. That's something a lot of BL's are still lacking in, but ULS is a very good example of how it should be done.
The OST for this is also very good. Gongchan and Seowon both have a song for this and I'm absolutely in love with them. The rest of the OST is just as amazing as well though.

TLDR: Watch it if you like a cute little love story with a bit of drama and suspense.