Pros: Detail, soundstage, build quality, comfort, weight
Cons: Bass quantity
I'll start of with the physical aspects. The headphones have a magnesium and plastic construction and feel light but well built. The 1/8 to 1/4" TRS adaptor is nicer than most and the cable is good (thick, no memory, strain relieved etc.) but single entry which I don't care for.
Fit is very good, the self adjusting wings work exactly as advertised and put very little pressure on any part of my head. Despite the very large size of the cups, the driver housing (not the driver itself!) still comes into contact with my ears, kinda like a on-ear phone. I don't find this to be a problem, it never bothers me and ears don't get warm in there. There are mods to correct this but I don't like the change in SQ.
The AD900 is midrange forward phone that is quite a bit on the bright side. However, the treble is clean and not at all harsh sounding so it is not as jarring as it can be in the Grado/Alessandros. The midrange is the highlight of the sound signature which I find to work quite well for the rock based music I listen to. It puts the guitars forward which I like but isn't thick sounding (for want of a better description) so it remains non fatiguing and doesn't detract from the rest of the range. Bass is tight and well controlled with decent punch but very little weight. This is my only complaint. Extension however, is very good. Soundstage is large but I never considered this to be an important factor, it is impressive though, if you are into that. The phones are very detailed and fast so they can easily keep up with complex music. The overall sound signature is quite easy to listen to, slightly warm. I would not call them neutral or transparent. Power requirements are low.
To sum up, a very nice phone. I listened to them almost exclusively with rock: indie, folk, acoustic, hard rock, extreme metal, punk, prog and some others. I find them to work well with these genres but haven't heard them with much else.