Re: how dusty does the Antec 900 get?
Actually the 900 has been better here dust wise over other cases. The fans themselves have turned out to be the dust magnets while blasting around the board and other items like video, sound, tv tuner, memory seems to see less on those.
On older builds the dust would build up along the sides(inside) as well as under the board along with the usual cling to things. Considering the increased front intake on the 900 and one ThermalTake model also seeing twin 120s in front the fans get the most.
Filters obviously provide a benefit while never eliminating dust 100%. They will cut the amount down to the greater degree however while a periodic cleaning won't seem as large a task.