Pros: Great sound, great comfort, changeable cables
Cons: Must be amped to perform. Much better with aftermarket cable
Bought these second hand from a fellow Head-Fi'er (thanks, Gavin!!)
These are easily some of the best value headphones on the market. Their sound quality and scalability (i.e. improvements gained through better cables, amps, etc.) is brilliant for the price.
I've tried them with all different genres of music and it doesn't matter what I throw at it - everything is effortless, smooth and creamy, but with incredible detail and imaging. The only negative might be a lack of thump with Hip-Hop/RnB. They have the bass, but not the boom. I find them to be just right, but bass-heads might be disappointed.
At 300ohms, they're not easy to drive so a high powered player (e.g. iBasso DX100) or amplifier is a must, but well worth the investment.
Full review: http://passionforsound.wordpress.com/2012/09/08/sennheiser-hd650/