I have it on lowest in Interface options. It's when you hold down M1 or M2 to look around or turn around with the mouse that sometimes it goes crazy high sensitivity for me and it is really problematic especially in PvP I reckon.
A strange thing is that when I have WoW maximized it often gets like this.
However when I reduce it by about 50% (in windowed mode) It gets all fine...
Really weird and it's probably not a 'server' issue but something with my resolution or smth I dno.
Anyone knows how to solve it?