First off, let's just get the negatives out of the way.  The MDRMA900s feel and look like $20 headphones.  I have no idea what Sony was thinking when they designed these.  They feel and look cheap.  The cable is also extremely long and thin.  It does not exude quality.  However, I suppose there is a bright side to the cheap materials used here - the MA900s are feather-light.  When I say light, I mean they are the lightest full-size headphones I have ever used.  They truly disappear on your head when wearing them.  They are one of the most comfortable headphones I have used and when wearing them I quickly forget how cheap they feel in my hands.
 
Now that the negatives are out of the way, let me get to the overwhelming positives - the sound quality.  As soon as I put these on, I knew they were something special.  The MA900s are very open and clear sounding.  To my ears, they have fantastic detail retrieval while not sounding the least bit harsh.  Vocals are very detailed while still retaining a full quality.  The bass is tight.  It's not boomy nor is it lacking.  Seeing as how these are open headphones, and I do mean open, the bass is actually a nice surprise.  I was expecting less bass but came away completely satisfied with the quantity and quality.  These truly are very well-rounded headphones.  
 
As for the soundstage.  It is expansive.  Sounds come from all around and instrument separation is some of the best I have ever heard.  Imaging also seems to be very precise.  All of this may be due to the large 70mm drivers.  I don't know.  Whatever the reason, the MA900s are technically very impressive.
 
All in all, the MA900s were a big surprise for me.  I bought them out of sheer curiosity and fully expected to return them when I took them out of the box.  That changed the moment I put them on my head.  
 
It reminded me that while it's nice to have a pair of headphones that look good, it's far more important for them to sound good and the MA900s sound very good indeed.