Public transport in LA.... well I have been on it and it can be used but it isn't the most user friendly. The bus stops are easily passed without noticing, and when you do find one you don't always know what bus it is for, finding information on connections is not easy, the spread of buses is pretty thin and I found (particularly Downtown) that the users can be a little intimidating (they weren't lying in Speed - locals only use the bus when they have absolutely no other choice). There is an underground metro network which I haven't tried but it is pretty limited. Also to note, there are several overlapping municipal services run by different companies. Basically the "city" is vast, sprawling, network and it extends over huge distances so the bus network is enormous, visible but not terribly usable. It is easy to have to travel twenty or thirty miles to get somewhere, so bussing places takes quite a bit of time. Also it just isn't pedestrian-friendly at all (a few places aside), street art is placed to be viewed through a windscreen for example. It is possible to navigate and get around on the bus but the hassle is not worth it for a few days. Hang onto the car, and "enjoy" the smoggy traffic if you aren't travelling alone you should be able to use the pool lanes on the free-ways. One thing to note, is that because of the increase in gas prices, the impact of environmentalism, the grinding traffic jams and a slow realisation that not all is right, LA is trying to improve it's [broken link removed]network... just not by the time you get there!!
I would second kkelliher, LA is not a patch on SF (haven't been to Vegas) but if your heart is set on it... stay out of Downtown (don't mind the advertising literature - unless you like the unedifying vision of rampant poverty, drug addiction, homelessness, etc right next to extreme wealth). Santa Monica and Venice Beach are worth a view. The museum district on the Golden Mile is worth looking at. Rodeo drive is worth walking down, Beverly Hills and the tour of the stars houses, Hollywood Boulevard in parts. Enough for a few days, enough to say you have seen the place but not my thing.