Pros: Great highs, mids, and comfort
Cons: Rapid bass response drop on low end
I've had Sony MDR-V6s for about 12 years, and I was looking for an upgrade. The Sony MDR-V900HD seemed to fit the bill, and I wanted to love these.
In many ways, I was not disappointed. Good comfort, isolation. The clarity of the mids and highs is exceptional. I can see why people that buy the V900s love them (or the MDR-7509HD, same headphone, different styling). Their highs and mids have a precision and accuracy that is alluring.
However, the bass response is lackluster. While I knew from seeing frequency response charts of the 7509HD that this might be a problem, I had high hopes that the bass response might be enough for me. Unfortunately, it's audibly obvious that the bass starts to roll off rather steeply below 100hz. And when I EQd the lower frequencies up to flatten the response, I didn't feel that the texture/detail of the bass response was up to that of the mids and highs. The bass detail was very good, but not great.
So while the phones are otherwise excellent, it doesn't make sense to me that Sony would design a monitor headphone for the digital age with the "HD" moniker that doesn't have a fairly flat--or at least more gradual sloping--low end response. There is simply more music than ever using the lower bass frequencies (e.g., compare modern/alternative rock with classic rock). And let's not even talk about movie watching on a pair of cans. The MDR-V6s can handle it. Why not a more expensive, revised model in their monitor series?
My final recommendation is an analogy: if you are the kind of person that is satisfied with the bass response from a great pair of 6.5" driver bookshelf/monitor speakers, than these could be an excellent choice for you. But if you are looking for the low end reaching bass response of a pair of floorstanding speakers, look somewhere else.