I belong to the minority of always inverting the mouse Y-axis.
To me, pushing the mouse forward means I have to look down, pulling it back, towards you must make the the camera look up. That's what makes sense to me.
Some people accept it as a matter of opinion, others think the logic is WRONG and that people who do it aren't human.
As a side-note however, I present you the following quote from Wikipedia:
After id Software's Doom, the game that popularized FPS games but which did not support vertical aiming with a mouse (the y-axis served for forward/backward movement), competitor 3D Realms' Duke Nukem 3D became one of the first games that supported using the mouse to aim up and down. It and other games using the Build engine had an option to invert the Y-axis. The "invert" feature actually made the mouse behave in a manner that users now regard as non-inverted (by default, moving mouse forward resulted in looking down). Soon after, id Software released Quake, which introduced the invert feature as users now know it. Other games using the Quake engine have come on the market following this standard, likely due to the overall popularity of Quake.
I guess since Duke Nukem 3D was one of the first FPS games I played more than 10 times through and since I used the mouse to aim on occasion (when you had to shoot certain objects that required careful aim), it might be the reason I use the mouse the way I do.
So how many inverters do we have here?