I purchased some of these today to try and come to my own conclusion of whether they are good or not....
My initial impressions are :
Bass - Quantity is very good although it is a bit monotone with not a huge amount of texture or detail, not terrible but not amazing.
Mids - These are quite recessed and nowhere near as good as my HD650
Treble - Quite good and pretty realistic decay etc. Just a bit metallic sounding and sharp and not as detailed as the HD650.
Overall I would say they are pretty good but once again I find my self going back to the HD650... Although the HD650 are a tad boring and lack sub bass the sound quality is just so much better. The only closed phones which have been comparable were the Denons although they hurt my ears... It seems that open headphones just sound plain better than closed ones for everything except bass depth and impact.
I find what you say about bass to be odd, as I have an open-backed vintage can with a RIDICULOUS amount of bass impact and sub-bass response. Many times more than any closed can I have, including the M50s.