For around 18 months the headset worked beautifully. It connected automatically to my phone and iPod and the controls worked perfectly. I was very impressed by the sound quality, especially when noise-cancellation was in use - something I found was necessary for clear phone-calls. The sound disappeared from the left-hand speaker without warning and the unit was returned to Sennheiser for repair under guarantee, the company quickly issuing a returns number.
It was two months before the unit was returned, instead of the 5-15 days suggested by their correspondence, and it was not fully functional. The sound quality was badly affected by clicks, whistles and creaks, apparently electrical interference from the connections because it could be made to occur when the headphones were moved even slightly. For a short while the problems disappeared and all seemed well but they returned - again without warning - and the unit is now unusable. Sennheiser has issued a second returns number but there appears to be no link to the earlier, low-quality repair since they require the same information I provided before.
If you are lucky enough to have one that doesn't suffer a failure you will be delighted but I'm unimpressed by Sennheiser's back-up.