The down-side to open back (if in headset variety) is that the sound leakage is easily picked up by the mic which can lead to a horrific feedback loop which can only be solved by reducing the volume or jury-rigging a closed back with tape, cardboard and chewing gum. This is a problem I've recently realised with my painful SteelSeries Siberia 'neckband' headset.

Also: don't buy neckband headphones!