Kinda expected. Halo 3 dropped 87% in Europe from 1st to second week.
You can't compare it to titles such as MK, which dropped 21% or Wii Fit which dropped 45ish. Fun fact; Gears of War had less a drop than Wii Fit in %.
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS