How can I increase the mouse sensitivity of Magic Mouse?
Here are 5 Mac utilities that alter the mouse acceleration curve, speeds, and/or add features to mice in OS X:

USB Overdrive

$20 (Shareware; Free unlimited, fully functional trial)


  USB Overdrive is a popular program
  that works with just about any USB or
  Bluetooth input device. It allows
  users to customize mouse acceleration,
  scrolling, clicking, and more. You can
  even configure per-application
  settings. A new version that supports
  Multi-Touch in Apple’s Magic Mouse is
  in development and expected soon.


Version 3.0.1 compatible with 10.4, 10.5, or 10.6

SteerMouse

$20 (Shareware; Free 30 day trial)


  Aside from mouse acceleration,
  SteerMouse offers a ton of other
  advanced features. Configure up to 16
  buttons, modify scroll wheels/balls,
  and set the cursor to automatically
  move to a specific location on the
  screen when a window is activated.
  Support for the Apple Magic Mouse is
  pending.


Version 4.0.1 compatible with 10.4, 10.5, or 10.6

MouseZoom

Free


  While MouseZoom does not alter the
  acceleration curve itself, it does let
  users bump tracking speeds beyond
  those available in System Preferences.
  An ideal solution? No, but it’s a free
  alternative that helps ease the pain.


Version 2.2 compatible with 10.1, 10.2, 10.4, 10.4, or 10.5

Mouse Acceleration

Free


  This Preference Pane is based on
  Richard Bentley’s MouseFix – a
  non-user-friendly command line
  utility. This version, however, makes
  it easy to increase mouse precision
  and speed up mouse movement with
  visual aids.


Version 1.0 compatible with 10.4 or 10.5

BetterTouchTool

Free


  This is a brand new piece of software
  that just came to be within the past
  couple weeks. It’s in beta and still
  buggy, but it packs some nice features
  and is in active development. I can’t
  say for sure whether it customizes OS
  X’s acceleration curve or just
  supercharges the maximum tracking
  speed. The real reason to give it a
  shot, though, is if you have a Magic
  Mouse or MacBook that supports
  Multi-Touch gestures. BetterTouchTool
  lets users assign specific actions to
  advanced finger gestures and can
  simulate a middle-click. It can even
  bring the Mighty Mouse’s Exposé
  functionality to the Magic Mouse.


Compatible with 10.6

ControllerMate

$15 


  ControllerMate is a controller
  programming tool that allows you to
  customize the behavior of your HID
  devices — keyboards, keypads, mice,
  trackballs, joysticks, gamepads,
  throttles, among others.
  ControllerMate’s philosophy is to be
  as flexible as possible, to provide a
  set of basic programming tools, and to
  allow the user to combine those tools
  in an endless variety of ways.


SOURCE: This was posted on Nov 24th, 2009 at MacYourself.com. Some utilities are probably updated now. I’ve used USB Overdrive with certain success.