This was really an exceptional drama! It's one of the best I've seen so far. Although the whole girl pretending to be a boy thing is not new, the setting and situation is fresh. This gives it a very different style than all the dramas it has been compared to. The beginning is slightly confusing if you have no knowledge of Korean history, traditions or culture. But as you watch, you will understand. I like how there is friendship built and emphasized as the characters encounter little twists and obstacles.
The characters play well off each other as their chemistry is greate and the actors are all well suited for their parts. And honestly, the guys are very cute :P
One notable thing about this drama is it gives you room to imagine in details yourself. For example, they don't necessarily give the full backstory of the characters. Instead they hint at many parts and let you create your own story for them in your mind.