you have to understand that propecia affects everyone differently. some people are super responders and it will not only stop loss from day 1, but regrow hair over time too. for others, propecia does nothing.
i would say for the far majority, yes, it stops loss on day 1. however, you will typically shed hair before it starts growing back. so while it may look like you are still losing hair from months 1 through 4, it will actually all come back in thicker.
at around 12 months you'll really see a difference so it is important to stay religious about taking that tiny pill once a day!