This is honestly a classic, and it was super popular for a reason. It's such a great drama.
The story is a tad bit cliche in places, but overall it is more unique and interesting than regular dramas. It's got great cinematics and writing.
The actors/actresses are phenomenal; every character is played by a great actor. The chemistry between the main characters is outstanding. The sad scenes are heart-wrenching, the happy scenes are so uplifting. It's just such a wonderfully made drama. The ost is great as well. You certainly need to check it out.

If you're just now starting dramas I would certainly recommend this one as a starter. If you haven't watched it yet you should.