These Sennheiser headphones are well constructed and the accessories supplied are good quality. Sound is good for on-ear cans although a little harsh on some music and lacking in smooth bass. The battery pack is removeable so if it fails I presume it can be replaced without replacing the whole headphone which is good. Pairing is quick and easy - I have paired it to a cheap bluetooth audio dongle and my HTC Desire phone (on which it can control track skipping).
My issue is with interference or electrical noise in the right earphone (the active one). The noise is not effected by volume control and switches off after a couple of seconds of pausing playback on my phone (presumably the headphones go into standby mode when no sound is being received). The noise is also present but sounding different when not using bluetooth and connected by the supplied cable.
The noise is very distracting, particularly as it is in just one ear, and listening to quiet sections in classical music would not be enjoyable.
I can't check whether this is a fault or is present in all sets - certainly my old Logitech BT headphones did not exhibit this. Could other owners let me know if they experience this problem?
UPDATE: Concluded my were faulty and contacted Amazon who supplied a replacement within 2 days - BEFORE I HAD RETURNED THE FAULTY ONES! This is the reason why I purchase so much from Amazon. The new headphones do have a very slight noise like a high whistle, but this is not intrusive and I have to listen hard to hear it.