He needs a proper noona to keep him in shape.
Like you say, gilnis, he seems like quite the male bimbo so if he's not careful, he's gonna get really old (as in uncool) really fast.
I trust him though. He's doing great so far and, although not long, he's been in the industry long enough to know a thing or two, I think. I mean, if it didn't get to his head bad enough to lead him astray in all the previous years when he was younger too, why would it happen now? I like his choices and where he's going with his acting, but he seems to push himself too much to please. Fans and producers alike. And both those groups of people can be ruthless. I wouldn't want him to end up seeing his job as a burden or feel like he needs to go to extremes to please because it's then when the acting and skills suffer.
5 years from now (heck, one year from now), no one will even remember he lost weight for YAB or care about it. And the girls going gaga over him now will have moved on. So pushing himself physically and going around people pleasing won't mean jack squat in a few years time. Good performances in great series/movies do though. It's those that people remember and that make an actor. I just hope he loves his art enough to go back to developing that after he's done raising a little hell and gaining publicity.
P.S: One Missed Call was terrible. Awful. I did like his performance though. Also, Keun Suk hasn't had big roles in the last 15 years. Modeling and cameos etc etc don't do much for ones skills in acting. As for Matsumoto, it's just my personal opinion. No need to feel offended.
I haven't watched many jdramas, indeed, but judging both from movies and series, I just don't "feel" japanese actors. They do have a certain style that I just can't get into. Maybe that's why I can't see Matsumoto's acting with a clear mind. The style gets in the way for me. I do hope to find better jdramas to watch though cause there must be good ones there too. But the ones I've watched (maybe bad choices) just don't make me want to see more. Just from what I've seen so far, very few japanese actors are of my liking. Whatever rocks your boat, they say.