After having spent a good amount of time with the TH-900, I can safely say it is my favorite dynamic headphone I've heard to date. I'll say that coming from the Senn 598/800, the bass on the TH-900s was extremely disorienting to listen to at first. It didn't murk up the music, but the sheer quantity compared to what I had been using was fairly overwhelming. It's definitely not for everyone, and I can understand why a few Head-Fiers whose opinions I highly value ended up disposing of their TH-900s.
As for me, It took a while to get used to, but the TH-900 does something special. Much like the HD 598, it has an addicting sound signature that makes a lot of the songs I listen to an absolutely immersing experience. Luckily, that's exactly what listening to music is all about for me. This was a very worthwhile purchase.