I felt like sharing some of my experiences on how the D5000 changed as time went by.
Now, I was never a believer of burn-in. I tend to be extremely pragmatic in nature - I only believe in something once I see it happen. I never noticed any burn-in over the year I wore the HD280 and the Porta Pro.
These are the headphones I own other than the D5000: An apple earbuds, the Koss PortaPro and the HD-280. I have not owned but have listened to the HD-650s and ATH-50s.
Before buying the D5000 I read a lot about the comments. I was frustrated with how bass-anemic the HD280 was and while I loved the PortaPro, I was curious to see how a more expensive pair would do. I read the whole MarkL Mod page and its main criticisms - that the bass was all over the place, too open, muddy, and taking over the low-mids. So pretty much the opposite of the PortaPro. Still, I wanted some high-end closed headphones and heard goods things about it. Specially the comfort. The HD-280 was absolutely awful for my head and one of the least comfortable experiences I've ever had.
When the D5000s arrived my first impression was - holy cow, these headphones are comfy. They fit perfectly, it was like there was nothing on my head. So I went to test it, hoping they would replace my HD280s. I was disappointed, to say the least - they were as bass-anemic as the Sennheisers. Compared to the PortaPro, they were still leagues behind. Even the apple earbuds seemed to have more bass.
The treble wasn't sibilant either. Now, this is important - I'm really sensitive to higher frequencies, so anything sibilant hurt my ears badly. I'm not very sensitive to the mids and high-mids so I usually have to EQ these up or else I can't understand what people are saying, and I don't care much for bass. The sound was incredibly smooth but anemic, and it didn't sound like an expensive pair of cans at all. However, I could watch my TV shows with it and it was great, the mids were very good.
Well, I started to wear them anyway. I do realize that a lot of the burn-in is psychological, so I felt like keeping the other headphones for different situations.
At some point, in fact, the sound became good. Still didn't sounded like top-notch, but it was satisfactory to me. The bass was okay, still open, but not very strong at all. The treble was good. I compared it to the other headphones I have and my written observations were about the same, so maybe I just got used to its sound signature and found it pleasant.
About six months later, I noticed some songs were a bit boomy. I listened to the Sennheiser HD-280 and it was definitely anemic. The bass was about equal in intensity to the PortaPro, but like MarkL pointed - too open and too muddy.
As time went by, I noticed the bass was getting progressively too overwhelming. The low-mid was getting overwhelmed by the bass and the mids overall were just weak. I'm here one year after first using the headphones, and compared to the other headphones:
Bass is TOO overwhelming. I read about modding it and removed the earpads and the fabric over the driver, and it greatly cleaned up the sound, but even without these and with Bass EQ'd to very low, it's still stronger than the HD-280s and all over the place. With everything in, the bass is so powerful that everything vibrates and makes a horrible sound.
Treble is very, very sibilant and I can't listen to music over a long period of time because my ears just hurt from the treble.
Mids are mudded and I can't really understand what people are saying. My observations remain the same as for the other headphones - I even got to test a brand new PortaPro a friend bought and compared to mine to make sure mine wasn't "burned in" and the sound was the same.
These changes started occurring in Month 3-4, became noticeable during Month 6, and have been increasing progressively since then until the point that the sound is horrible right now. When I compare the headphone with the others I own I get the exact opposite impression from when I bought them, except maybe for the bass, it's always been open. Anyway, hope this helps someone or was, at the very least, entertaining/informative to read. By the way, my hearing range hasn't changed from the last year (still very sensitive to high frequencies).
I take everything subjective with a grain of salt and always recommend the same, as our brain can play tricks on us. Still I can't help but feel like some real burn-in happened and it wasn't positive at all. My end impression of the D5000 are the same as the many people with complaints about these cans.