Packing just two platters, the WD400BB is indeed a quiet and cool design. Active cooling should be unnecessary in most cases. Even when seeking, the drive is quiet. Though not as silent as units from Fujitsu or Samsung, the drive is nonetheless quieter than recent units from Maxtor, themselves not exactly roaring disks.
The 400BB's WinMark scores shatter previous records... they stand in a class of their own. IOMeter scores, though not staggering, represent improvement over previous models (and some of the competition, for that matter). When combined with quiet and cool operation, such performance makes WD's unit a viable alternative to the Quantum Fireball LM (fast but noisy) and, depending on final pricing, the Deskstar 75GXP (fast but relatively pricey). Those looking for a drive smaller than 40 gigs should consider it.