BRXIgnition... The sound leakage is actually VERY good. I use them in a place of business and have two co-workers in the same room about 8 feet away. I turned up my iphone and laptop to 80%ish and they couldnt hear it at all. Tested at home too and even 5 feet away you BARELY start to pick up any sound. I returned two different headphones for this very reason. These are by far the best that I have used in a quiet environment.
That said, to my ears, the mids and highs are very smooth and well balanced. Not too bright, but very natural. As you increase volume, both stay well balanced and distortion is minimal. I've tested classical, opera, 40's/50's, instrumental, pop, rock, metal, rap, r&b, and dub step. The M100's handled all of these very well. The dub step was the only one that the bass was not as good as I had hoped. The bass is tight and sounds good with alot of the music I tested, but with dubstep the lower frequencies lacked the depth and power that I wish it had. I actually went to the audio store with my headphones and listened to the $300 and under gamut they had along side mine. (And yes burn in is still in progress. I've had them running for about 14 hours). The verdict for me was that I had the best headphones I could buy for $300. I do wish the bass had more depth, but it is better than satisfactory for most music styles.
To echo what's already been said, build quality is excellent!! Very small form factor, light weight, comfortable and they look killer. Love the cables and the ability to plug into either side of the cans. The orange cable is rockin. I chose the black model and couldn't be happier. I haven't tested the accessories yet. The M100's are definately keepers in my book. I wouldn't hesitate to buy them again.