In this case, an answer like 'I'm not sure" means that the person hasn't made up their mind yet. They have experienced driving downtown, and didn't hate it immediately, but as to whether they actually like it....well, they aren't sure yet. It may take a bit more time for them to arrive at a definite position. In this case, the person is being asked about their feelings, and they can't say for certain what those are yet. In the second case (how many public holidays in China), it is a factual question - there is only one right answer, and you either know it or you don't. It doesn't have anything to do with how you think or feel about the idea of public holidays. So when someone answers "I am not sure" to a question that has a factual answer, they are basically saying 'no, I don't know". They might follow up with - I am not sure - maybe ten? Other times people say 'I am not sure" as a way of avoiding having to give a direct answer that might offend. In these cases, if they say "I am not sure", it is most likely that the answer is 'no', but they wan't to avoid saying that, so prevaricate with 'I'm not sure'. Personally I can't stand it when people do that - but there you are - people can be funny (as in strange, not amusing)!