m_filter does nothing more than comparing two hardware inputs from the mouse.
And it adds a third by software to make the crosshair look more smooth and accurate, and that is the average between those two hardware inputs.
However, if you use acceleration and/or high sensitivity, the crosshair may feel choppy. Like it almost lags. That is when m_filter can help out, and make the feel of it more smooth and look a lot more accurate.
That's why some q3 players use it, like cooller, lexer. Last guy uses cl_mouseaccel 2!
But since we are talking about slower paced games like RTCW and ET here. And most ppl use low sens and no accel, m_filter would only make your accuracy a lot worse.

To sum it up:
High sens : m_filter 1
cl_mouseaccel: m_filter 1(if it looks choppy)
Low sens: m_filter 0

I personally recommend m_filter 0, because I like the mouse to show the exact mousemovements I do onscreen by hardware. And I am a reflexshooter on low sens. So it would affect my aim to get a lot worse in reflex shooting, or flick-shoots as others call it.
But experiment to see what suits you the best.

 

이티같은 느리고 저센시를 많이 사용하는 게임에서는 해로울 뿐인것 같다는 자신의 주장이네요