I finally got my pair of 900's today, after waiting patiently for a week after the estimated delivery date...
My first impressions are that they're incredible, and a perfect super-step up from the xb500's I had. I'm not noticing the sibilance people mentioned at all, but I can definitely hear the V-shaped frequency curve. Every sound and instrument sounds clear and detailed, and it doesn't feel muddy at all. In fact, I don't think I've ever heard such clarity, and every song I listen to sounds completely different.
I've tried every genre with these headphones, and they're perfect for electronic (house, dubstep, etc..). The bass has great impact and rumble, and is really tight and satisfying. Hip-hop and R&B are also quite good, so are some bands like Pink Floyd, Radiohead and Coldplay. However, I find some rock difficult to listen to since it's centered around the mid-range, and these headphones put an emphasis on parts of the songs that aren't intended to be emphasized, such as hi-hats and bass rather than vocals. Thankfully, I've been growing out of rock for some time now, and I'm likely not to use them for it much.
Currently I'm running them just out of my iPhone, but I'm due to get a portable amp (Fiio e11) soon. I'm noticing that they aren't very loud, and it's probably because they aren't being driven that well by my phone. Nothing seems distorted when I crank them up though, and I'm beginning to wonder if an amp is even necessary.
Anyway, those are my first thoughts. Glad to be part of the Pro-900 gang now!