Hello carlove,
I'm listening to some J-Pop through these headphones right now, and can tell you that they sound very good to me. They are a bit bass heavy, but not overpowering, (at least not to my ears). I tend to use these with my iPhone 4 and iPod classic, and they sound very good. I've found them to have a laid back sound signature when used on their own, but really open up when used with an amp. I would suggest you try to give them a listen at an Apple Store if you can find them. Overall, I like these headphones. The only other full size cans that I can compare them to are my Sennheiser HD-580, and compared to that classic, the L1 sounds pretty good. They have more bass than the 580s', and the midrange is good, but the 580s' have a wider, deeper soundstage in my opinion. I can't compare them to any of the more current models out there at the moment, but I would suggest trying to listen to Sennheiser's new Momentum Headphones. They are brand new, and many here have said they have a brighter sound compared to the L1. I would try to listen to both and see how you feel about them.
As far as sound leakage, the L1s' are semi-open headphones and they do leak some sound, but I've found leakage to be fairly low. Happy Listening!