Only weakness I've had is the volume dial. My original set (360s) started to crackle and then eventually one or the other speakers would sometimes cut out when I changed the volume using the dial. Sennheiser replaced them in less than a week just inside the warranty period for a brand new set, which were perfect at first but have recently started to crackle sometimes in the same way.
As I very rarely change the volume using the dial it's not a massive pain, but it is obviously a weakness in design or manufacture based on my experience of these 2 sets. Aside from that however, they have been flawless, with no obvious signs of wear and peerless quality (for a headset - I appreciate they are not high end audiophile headphones).