I love the Metro. But only if my destination was conveniently located near a Metro station. See, that's the problem with our rail system. The routes/stations are somewhat limited and spread out.
There's no direct route from LAX to DTLA. From the airport, you have to take a bus or a shuttle to a Green line station. Then get on the Blue line to DTLA. The Blue line does go through some rough neighborhoods. Maybe that's what your friend is referring to. But I've never had any problems. As long as you go during the day, there are plenty of other people on board. And cops do patrol the trains and stations. Just don't ride in the late evenings. And don't look like a tourist.
Get the day pass. I think it's still $5. Unlimited rides/routes. If not, then you have to pay the fare for each line you get on, each way. LA traffic sucks. I would only recommend driving if you needed to go somewhere that wasn't near a Metro station or bus route connected to a Metro station.