I watched several times recently as they were boradcasting it again at different time of the days. Well, the reason I watched it was I was bored and looking for something to do, other than spending time on my desk looking into computer screen. The 2nd reason was that I liked the story, progress of their practices, human life like friendship, love, and so on. (I know it's all made...but)
I only watched Part 1, the part 2 was not that interesting, compare to the first one as I think those producers should've made more changes to the story... And I liked the music used in that drama. I don't know what song it is...but it's kinda sawayaka...
I have seen some of it since my host family is obsessed. As to why it is popular... Let's see, it is a drama, it is in Japanese, and it has a bunch of well built guys running around in speedos...so it has a rather large fan base.