Faulty, and Sennheiser won't replace them

If they worked properly they would be great.
But after a month they started emitting a very loud buzzing noise when in noise-cancelling mode. This noise is almost deafening, so it means I can't use the noise-cancelling feature at all. I returned them to the supplier, who took 3 months to decide there was nothing wrong with them and sent them back. Or so they say, since I haven't received them yet.

I will never buy Sennheiser again.

Recent update: After writing a strongly worded letter to the retailer, my Sennheiser earphones have been replaced. They are so far functioning normally, but we'll have to wait and see if they go the distance.