Overall - Tumbling was the Drama that got my current j-drama obsession started - that's not to say that it was my first drama though. This only makes it more spectacular in my eyes since I had little interest in Drama and only watched a few to understand what a few friends were talking about - until I stumbled upon this beauty.
This drama actually got me into the manga, which is rather backwards...
Story - Each character has a backstory and /grows/ throughout the series (except the background characters that only have 7 lines in the entire drama) in a way that reminds me of a novel, almost. It shows that they were developed, 3-dimentional characters that seem all the more real because of those facts. This is major kudos to the writers and original mangaka.
Music - I never focus much on the background music in movies or dramas unless it sticks me as wrong for that moment. I do know that they used the theme song throughout the story, which was interesting because it fit in every moment they used it in, even though they were vastly different. this part comes with a warning; you WILL have the song stuck in your head for a good while if you watch the episodes in a few (1-3) sittings. However it's a good song, so I didn't really mind.
Rewatch Value - I've already rewatched this drama once, my favorite parts about a million times, and created stories in my head for when I'm bored and want to play with ready-made characters. (Can't tell I'm a writer, huh?)
Acting/Cast - Yamamoto Yusuke is really just the cherry on top, in my opinion. His acting skills are so great, and he covers so many different rolls so efficiently and /well/ in different dramas that its just inspiring. He became an actor I follow after Tumbling and, after a few more of his dramas, he became my favorite actor. He's just amazing.
I feel I have to mention the others (that way it doesn't become another of my Yamamoto-ramblings). They cast this very well and everyone fit the part and acted their rolls naturally. Some of the actors were better than others (the Mizusawa character faded into the background too well and was rather emotionless unless the situation called for extreme emotion) but none of them were 'bad' actors, just differing styles.
Really, really worth the time to watch, rewatch, think about, and drive that damn song out of your head until the next watching!