GRE: how important is it to get it in time?
Three things: 


the application deadlines for you are several weeks earlier than the dates that decisions on the applications are made, so even if the scores arrive after the deadlines, they may still arrive in time to be taken into consideration. 
different U.S. disciplines (and to a lesser extent, different departments in the same discipline) have radically different views on how much the GRE is worth. So the answer to your question really depends on what subject you're intending to study, which you haven't told us.
bureaucracy at some institutions may prevent your application from being considered, even if the faculty thinks the GRE is unimportant. 


The best thing to do is write the schools you are considering and ask. You may have taken the GRE in time for some of them, and I expect others will consider your application even without the GRE.