I have the Superlux HD681, which are pretty close to the HD668. They sound great. The amount of sound they leak is modest, but they are definitely not great as far as isolation goes. In general, I'd go with in-ears if isolation is important to you. If you can't tolerate in-ears, a good pair of closed headphones like the ATH-M50 will do well, but you'll have to pay more $ for the same sound quality (at least in my experience).
