Pros: Airy, extended highs, good wearing comfort, balanced signature, excellent build quality
Cons: Slightly incoherent / smeared soundstage, bass roll-off, can sound aggressive on some badly mastered tracks
Along with the summary below, I have posted a comparison review of the AD900X, AD1000X and MA900 here:
I've also got a youtube review of the AD900X. If you like the video check out my channel for more reviews :)
The AD900X has excellent build quality (astounding for the price) and great wearing comfort provided the 3D wing design works for your head.
The AD900X has an airy and spacious sound with a particular emphasis on mids and treble energy, though it is not overly analytical because of a good depth and decent extension in the bass. However it lacks a little definition and on densely layered music it starts to sound incoherent or inarticulate. Bass is tight and fast but does not have the same rumble or depth as closed headphones or some orthos. On slower music (say acoustic or slow jazz) the AD900X sounds fantastic. It loves stringed instruments as well. On badly mastered tracks the treble can be too aggressive.
With the Japanese price of the AD900X and with mail forwarding set up, the AD900X is a veritable bargain. The sound is engaging and exciting. Personally I upgraded to the AD1000X and I think for not too much extra money the AD1000X is easily the better headphone, but the AD900X is still pretty awesome.