It really depends on the context, but in every case it is very informal. You can't just use it as a synonym for "girl" or "lady". If you use it in the wrong situation, it could be rude. I've heard it used in these situations.
1. Men (guys/boys) use it to talk ABOUT a woman. "That's a hot chick."
2. A man or woman might use it in a derogatory way. "I'm sorry I'm late; some chick was holding up the line at the grocery store."
3. People don't use it to talk TO (address) a woman. If you do, it would probably be in a mean way. "Listen, chick, move your car now or I'll call the police."