seriously? a decent mouse will help to a certain degree, but your own abilites are usually a major factor on aiming
was a big counter strike player many years ago (before the addiction of consoles got a grip on me) and originally used a bog standard mouse. eventually went to a microsoft 4 button optical mouse, which was quite a step up. yea the mouse helped, sensitivity etc, but aiming abilities were pretty much down to me.
tho since i've been outta pc gaming for many many years things may have changed greatly so this is only my own, rather uninformed opinion