Summary: I purchased these headphones in August of 2010 as an upgrade from the 7506. The audio quality is definitely an improvement and the pads actually encase my ears fully which cuts out a lot of outside noise. This is huge when mixing in a large venue. The long cord allows me to move around to different racks on a gig or in the studio without having to always set the headset down. Now here's the bad news.
Summary: I previously owned the MDR-v600's I bought 10 years ago. To replace those. I bought these. I tested a friends MDR-v900's and I loved them. Part of the reason why I'm particular about the sony MDR line is because unlike other users, I'm not partial to bass. So just haveing a deep booming bass, doesn't qualify to owning a great pair of headphones. So lots of people will say this pair of headphones isn't very good.
Summary: I use these primarily to record a female vocalist. These headphones are perfectly comfortable. Their sound is excellent. The 7506s are very bright (which some vocalists like) and tend to bleed in the microphone. But these 7509 HDs are accurate with virtually no bleed. The bass is tight and the highs are smooth. Couldn't find any cons. I highly recommend these headphones.
Pros: Comfortable, Durable, Good Bass, Good Value, Great Sound
Summary: I listen to a lot of music and talks/lectures/interviews on computer, iPod, and portable CD player, using a pair of Sennheiser sport earphones which I love. I bought the Sony 7509 headphones specifically for listening to meditation tapes which require lower frequencies (5 Hz) than can be reached by the Sennheisers (which just go down to 13 Hz).
Summary: The Sony MDR-7509HD phones look identical to the consumer Sony MDR-V600 headphones but they sound much better. Smoother high frequency response results in less listener fatigue compared to the excessively bright sounding professional standard MDR-7506 phones, and larger ear cups provide better isolation. The slightly larger physical design eliminates those pesky little driver connections on the MDR-7506 phones.
Summary: I listened to samples on a lot of headphones and when I got to the 7509-HD it was like a whole new listening experience. the level of separation was incredible. This is what I was looking for. I could hear the sounds from every instrument in a recording. This may be good for my hearing but you really have to try on different earphones and find the one that sounds good to you. these exceeded all my expectations.
Pros: Comfortable, Durable, Good Bass, Great Separation, Rich Sound
Summary: At the top end of the Sony professional headphone series, these offer smoother high frequency response than the venerable Sony MDR-7506 and MDR-V6 models. Improved drivers are installed in what appears to be identical or very similar headgear as Sony consumer MDR-V600 phones. The large ear cups provide greater comfort and improved isolation than the MDR-7506.