Pros: price to value, airy sound, comfort
Cons: bass, slightly muddy
These are great headphones for quite a budget price (at least in Japan and America).
I also own the AD2000 and the W1000X so here is a little comparison:
The W1000X would be the warmest sounding of the bunch. The AD900s openness also takes away something from the bass area that would be needed for a warm and embracing sound. The AD2000 is airy as well, but a lot closer and tighter than the AD900. In direct comparison the AD900 seems to be rather muddy with not quite as clear mids and highs than the AD2000 or W1000X. Whereas the AD2000 doesn't seem to be needing any amplification (of course it scales well), I would highly recommend to amplify the AD900. Comfortwise it's en par with the W1000X or maybe even better, considering that the W1000Xs pleather pads can get pretty hot on the ears. Listening to the AD900 I can understand people complaining about a certain detachment from the music, as if one is floating in outer space with the music being everywhere. If that is not your cup of tea, better stay away. All in all the AD900 is extremly non-fatiguing even at higher volumes. The W1000X has to be seriously EQed before being comfortable for longer sessions and even the AD2000 needs some adjustment at higher frequencies for that. The slight muddiness of the AD900 on the other hand let's you dive right in without any adjustments at all. I like Audio Technicas adapter to 1/4 a lot, it's a sturdy unobstrusive construction.
I can highly recommend the AD900 to anyone listening on a budget and preferring a translucent and open soundstage.
Great for classical music, might not work as well for close quarter chamber music or tight cellar jazz recordings though. It's pretty good for electronic stuff as well, mind the lack of bass though.