Mantra for tourists (especially those of you who come from localities where there is good public transit): L.A. has a VERY rudimentary transit system that is not rapid, and does not serve most of the places you will want to go.
If you are visiting friends, family, or business associates in L.A. and you admit to them you are using public transit on your visit, they will (rightly) perceive you as crazy.
Your time is valuable.
Rental cars are cheap (can be obtained on Priceline.com at LAX for typically $16-20 per day).
Repeat after me: Dude, you need a car in L.A.