You're right :) It's mostly used when a person means "ok", but can also be said when you like something. It's very casual, everyday language and should never be used in formal speech. *It also has one more negative connotation which refers to drugs, or anything that is prohibited; if you want to use the phrase in this way, you must be sure to say it with proper intonation so that people will understand what you mean. Examples: "I'll meet you at the cinema at 6 tonight." "Good stuff. See you there." "Have you tried this spaghetti? This is some really good stuff!" "All the medicine sold in the stores doesn't help my headaches." "Me neither. But you know they (store owners) keep the good stuff in the back (in the 'employees only' section of the store where it's not accessible to customers) anyway."