Beautiful Days is one of the best Korean dramas that I've watched so far. It has a good plot, good characterizations, great acting by the leads (CJW, LBH, RSW, Lee Jung Hyun, Shi Min Ah), and excellent OST! Though the story slows down a few episodes toward the end, it picked up pace again. I also like it because it focuses not only on the love story, but also on friendships and family relationships. Some may not agree, but it reminds me of some of the best HK dramas of the 90s such as "Greed of Man", "The Big Family" (Da Zhia Zhu), "Wo Pen Shan Liang" etc.
I would like to add that LBH is really excellent here. He's one big factor why the series has a big impact on me!