Of course it is a science, but most critics of it should remember that it is a social science, and comes with all of the difficulty of proof that all social sciences suffer from. There are some common principles of the field that have strong theoretical and empirical foundations, and the rest of them are trying to make do with the constraints forced upon them by virtue of working with humans.

The principle of Supply and Demand is one that has so much historical empirical evidence that it might as well be called a physical law...but we can't call it that because the principle relies on human cognition, culture, and other non-deterministic influences. Trade theory also has an extremely strong empirical evidence. A lot of microeconomics is also very emprical, and even has a lot of experimental control mechanisms for which macroeconomics does not. Still, we can accept these ideas as true beyond any reasonable doubt. However, we have an extremely long way to go before the rest of the field catches up to the same standards.