For any serious ‘competitive’ gamer - I advice switching OFF mouse acceleration. As a long time CS player, this concept is drilled into you over and over again. There are even registry mouse fixes available from the CPL to ensure all forms of in-game and windows mouse acceleration settings are disabled.
I don’t think anyone has said why as yet, so I’ll attempt to explain.
Over time you develop a natural reflex in games - especially first person shooters. You want this natural reflex to map to the game exactly the same way, every single time. With mouse acceleration enabled, this reflexive action is hindered by software adjustments to the mouse input.
Your cursor should move to the same position on the screen each time no matter how fast you move the mouse. You should only be relying on distance of mouse movement - not the speed - for determining where your aim goes.
Let me try to visualise it for the readers.
There is a corridor 10 feet wide (in-game) which maps to say 10inches on your mouse pad. If you have your sites set to the corner of the left wall and someone appears at the right corner, you move your mouse 10 inches to the right and fire.
If you have mouse acceleration turned on - how far and how fast do you move your mouse? It’s an added element of complexity added to the processing your brain has to do - and usually it’s a best guess attempt only improved (but never perfected) with constant practice.