How feasible is it to use public transportation as a means of useful transportation?

I'm debating moving to either NYC or LA after I graduate (from High School). One of the biggest draws of NYC for me is the subway system. I definitely don't plan on driving. So could I use the LA public transportation system (i.e. subways, light rails, etc.) as reliably (or at least close enough to) the NYC system? I would be needing to get to and from work, as well as to and from school in addition to just getting to and from places with friends. Without having to spend a half hour waiting for the train/subway that is. And still get there in reasonable amounts of time.