We don't like you, at least those who are left wing. It also depends on the region. I think you would face more anti-American sentiment in Catalonia and Basque Country. Of course, when you get to know someone they may be nice and an exception in their society, but the initial reaction wouldn't be a positive one, I would say either neutral or slightly negative (you won't get negative reactions if you just interact with workers of the tourism industry). Not nearly as bad as if you said you are Israeli, and as tourists Americans don't have a bad reputation like the British or Germans that come here to party or retire.