Depends on how loud you have your music. I played some Black Flag at a bit above what I'd call normal volume levels and put it on my leg I could hear some sound like music in the distance (this is from about 50cm away though. If I cover the cups I hear nothing. So yeh they leak a little.
Next test I cranked it up to louder than most people would (especially if they care about their hearing). Placed them flat on my desk and sat in the desk nearby mine. I could just hear it.
So yes they leak sound, but I wouldn't say there's a "lot of sound leakage". Not as much as fully open back headphones of course, but if you want to leak no sound at all, I'd go for something different.
Edited by Stealthy Ninja - 12/2/13 at 5:35pm