After reading reviews on DYLB?, I thought I would really hate this drama. Quite on the contrary; I do believe that there are some shortfalls in the plot, but the depth of the characters and the music make up for it.

What I liked:
As a violinist and pianist myself, it is always disappointing when dramas don't make an effort to accurately display musicians playing (e.g. the bowing doesn't line up, the pianist is just hitting random keys, BOF I'm looking at you!). I therefore would like to praise DYLB? and the characters for attempting (quite successfully, I might add) to actually play the pieces. Although some might consider it a small detail, it just shows the dedication of both the characters and the director as well as contributes to the immersion into the story.
Second of all, the music was top-notch. I think DYLB? did a great job at selecting pieces that pertained to the Brahms/Schumann story, and while these served as plot devices, I enjoyed the traditional departure from traditional OSTs for dramas. The classical music was well performed, and I sometimes go back just to listen to the OST just because how good it was (which I rarely do for any drama!). It is also a great drama to introduce the world of classical music. I think a lot of people believe that classical music is boring, but this is definitely not the case.
The characters were also very well written. Each character had its own complexity. When I was watching this drama, it felt like I was watching real humans. I could really sympathize with each character.
Then, let's get to the acting. I think Park Eun Bin and Kim Min Jae did a superb job at acting as a musician. Their acting was excellent and sensible. PEB did a great job at portraying the struggles of a music student, and KMJ did a great job of portraying a pianist, and I felt their performances were very believable and enjoyable.

What I didn't like:
As with most of the critical reviews, most of my issues stem from the plot. I felt that the plot was a bit too slow, and I would've liked to see more plot development in regards to PEB in music school and KMJ as a pianist. I know that the main purpose of this drama is romance, but it felt like the characters were stagnant in a plot that almost didn't exist. I didn't really see any improvement or plot movement in regards to PEB in music school, or how KMJ became a more mature pianist/dealt with his relationships with professors, etc.
Another problem is the ending, and this is why I have marked spoilers. I felt that the ending, contrary to the whole story, was quite rushed, and they left a lot of open holes. The ending left me unsatisfied, as there wasn't sufficient closure for some of the characters. The ending was also very rushed, which didn't allow me to appreciate the ending.

Overall, a very great drama which is a bit underrated on MDL. I think the reason behind this is its slow pace might be a bit off-putting for some people. If you are intrigued, please watch it, I don't think you'll regret it!