Same problem here - with 2 WD15EARS-00Z5B1 - under linux (2.6.31) and ext4 fs - system is irresponsible and I can’t do two task or more at once (for example copy something and using finch / rtorrent). There are huge latencies while using terminal 
I hope WD will fix this by firmware or I’ll have buy two Samsung F2EG and sell this creepy WD 
sigma cpu # hdparm -i /dev/sda
/dev/sda:
Model=WDC, FwRev=80.00A80, SerialNo=WD-WMAVU1249613
Config={ HardSect NotMFM HdSw>15uSec SpinMotCtl Fixed DTR>5Mbs FmtGapReq }
RawCHS=16383/16/63, TrkSize=0, SectSize=0, ECCbytes=50
BuffType=unknown, BuffSize=unknown, MaxMultSect=16, MultSect=16
CurCHS=16383/16/63, CurSects=16514064, LBA=yes, LBAsects=2930277168
IORDY=on/off, tPIO={min:120,w/IORDY:120}, tDMA={min:120,rec:120}
PIO modes: pio0 pio3 pio4
DMA modes: mdma0 mdma1 mdma2
UDMA modes: udma0 udma1 udma2 udma3 udma4 udma5 *udma6
AdvancedPM=no WriteCache=enabled
Drive conforms to: Unspecified: ATA/ATAPI-1,2,3,4,5,6,7
* signifies the current active mode
sigma cpu # hdparm -i /dev/sda
/dev/sda:
Model=WDC, FwRev=80.00A80, SerialNo=WD-WMAVU1249613 Config={ HardSect NotMFM HdSw>15uSec SpinMotCtl Fixed DTR>5Mbs FmtGapReq } RawCHS=16383/16/63, TrkSize=0, SectSize=0, ECCbytes=50 BuffType=unknown, BuffSize=unknown, MaxMultSect=16, MultSect=16 CurCHS=16383/16/63, CurSects=16514064, LBA=yes, LBAsects=2930277168 IORDY=on/off, tPIO={min:120,w/IORDY:120}, tDMA={min:120,rec:120} PIO modes: pio0 pio3 pio4 DMA modes: mdma0 mdma1 mdma2 UDMA modes: udma0 udma1 udma2 udma3 udma4 udma5 *udma6 AdvancedPM=no WriteCache=enabled Drive conforms to: Unspecified: ATA/ATAPI-1,2,3,4,5,6,7
* signifies the current active mode
sigma cpu # smartctl -i /dev/sda
smartctl version 5.38 [i686-pc-linux-gnu] Copyright (C) 2002-8 Bruce Allen
Home page is http://smartmontools.sourceforge.net/
=== START OF INFORMATION SECTION ===
Device Model: WDC WD15EARS-00Z5B1
Serial Number: WD-WMAVU1249613
Firmware Version: 80.00A80
User Capacity: 1,500,301,910,016 bytes
Device is: Not in smartctl database [for details use: -P showall]
ATA Version is: 8
ATA Standard is: Exact ATA specification draft version not indicated
Local Time is: Fri Jan 22 22:48:30 2010 CET
SMART support is: Available - device has SMART capability.
SMART support is: Enabled
