100000% recommended.
The plot was great but the ending was so bittersweet. However, I'm still very glad I watched the drama because it was so unqiue and interesting.
First of all I'd like to say that at first I didn't appreciate Sung Hoon because of how much I appreciated Sung Joon for being nice and sweet. But as I found out more about Sung Hoon I realised how great he is too. I would have loved more twin/clone time and I think the (spoiler ahead)

death of Sung Hoon was unnecessary....
Well, not entirely. If Sung Hoon was seen to die but then at the end found out to be still alive, it would be such a fucking great ending.
Sung Hoon and Sung Joon never got to spend any time together and when they did it was always in the worst situation. I really loved this drama and I am very glad that Mi Rae, Chief Jang and Soo Yeon are alright. I am very very glad that Sung Joon is alright too.
However....Sung Hoon....I feel as if he could have had a better ending.
Overall, though, I loved the drama and I shall remember Sung Hoon as Sung Joon remembers him. Maybe I wouldn't rewatch the ending...but it would be great to see the twins/clones in action again.
Much love to this drama.
Hope to see similar ones in the future.