Cons: Sound leakage.
I've had these cans for over 10 years and they have been great. Super comfy to wear - I can wear them for 2-3 hours at a time. Rather bulky if you have to cart them around. The sound is typical Sennheiser, clean, crisp and fairly neutral. Lacking in bass possibly. Lovely delicate highs. Being open backed, they leak sound like crazy and used to annoy my colleagues in the office :)