Very complimentary. One is open, one is closed. Similar sound signature, though the Mad Dogs are a bit more engaging, a bit more PRAT (which boils down to "makes my foot tap" to me), while the HD650's have a bit more breadth to the sound stage, are a bit more polite. Both are similar, both can be listened to for hours, both are great headphones. I own both, and plan to keep both. If one had to go, it would be the HD650, though more because I need closed headphones, and less that it is "worse" in any way. That said, I prefer the Mad Dog by a bit, and they get the most head time by far.