I usually turn it on, I like it if it's done well. But motion blur and AA I always turn off, even if I have enough power for them. --- He who stumbles around in darkness with a stick is blind. But he who... sticks out in darkness... is... fluorescent! - Brother Silence
Motion blur and DoF I always turn off if I can. It just looks bad 99% of the time.
AA I keep at 2x or none; too much processing power for far too small an effect.
Bloom and HDR I actually like, oddly enough, so they're usually maxed in any game with options for them.
Shadows I normally set to low or off. Very few games allow shadows to be enabled with little to no performance hit, so they're usually the first thing to go when a game performs sluggishly,
By far the most important thing though is 60+ FPS. If the game is running below a constant 60 FPS (or, in specific cases, below the intended max framerate *coughcoughDarkSoulscoughcough* ), it's time to lower some more settings until it's running at a smooth 60+ FPS.
Its meant to simulate the focus of your eye, but being that they can't track what you're eye is actually focusing on, it more often than not looks like crap. This and motion blur and sometimes AO (except HBAO) are the only features I disable. --- Steam ID - BarneyRound