Spent several hours here today. It's free and I had down time on this business trip so why not? Charlotte's airport is reportedly the 6th busiest in the nation. That's very believable. If you're into seeing a lot of different plane types or planes from many different air carriers than this probably isn't going to be as interesting. Charlotte is an American Airlines hub and basically 90% of the traffic is theirs. Charlotte also has no direct overseas flights so basically what you'll see are Mid to smaller regional size aircraft in the AA fleet and much the same with the other carriers.
It's probably best to go when the wind has a southerly component to it so the planes are taking off and landing going south. When it's the other direction you're positioned on the North end of the runaway away from a lot of the best takeoff and landing action.
There's trash cans and park benches there with plenty of parking. There was a police presence there much of the time I was there so you feel safe. All in all a great place to enjoy a lunch or leisure time or take the kids.