I want to augment what has already been said here. People who don't use it to commute definitely have the impression that it's only good for going to sporting events or the airport, but really it is excellent for commuting if you live and work near train stations. During commute time, it runs often and fast, and is filled with commuters. Those who only use it during the weekends get the impression that it's only for "tourists and lower income demographic" (as Hannah put it), but during M-F 6-9a and 3-7p it's packed with business people and other work commuters and is much faster than getting stuck on the surface streets.
To give you an anecdote: I live in Reynoldstown about 5 minutes walk from the train station here. When I worked in Midtown, it took me 25 minutes each way on the train. Driving, was 30 minutes in the morning and ~40-60 minutes in the evening due to traffic. That is even counting the fact that I had to transfer to a separate line in my rail commute.
My opinion on the underlying question asked though, is it's much more convenient to have a car. Especially if you don't live and work near a train station (not a bus stop, a train station). But, even if you do, you'll find that few non-work hotspots are connected by MARTA. It's doable (I lived with just a bike and MARTA for 5 years), but not comfortable.