"Ally" is probably too weak a word for the relationship we have with Israel. Probably no other country has this much influence on our politics. Not even the UK or Canada, who we share close cultural and linguistic bonds with.
Israel is like the Taiwan of the Middle East, meaning it's a useful partner to us in terms of maintaining a separation between the West and some perceived enemy (Muslims in this case, the Chinese in the case of Taiwan). But they also cause us to have more enemies than we otherwise would. In an open letter, Bin Laden specifically called out our support for Israel as one of their justifications behind 9/11.
Spying between allies is a relatively normal thing, but when someone screws up and lets the public see it happen, usually there is at least some mild chiding. Israel doesn't even get that tsk tsk because of how much influence they have.
Also, Israel has a very effective propaganda machine that purposely tries to paint any criticism of Israeli policy as anti-semitism. It's hard for our politicians to openly discuss them without risking their ire, and that of many influential Jewish Americans, and the powerful AIPAC lobby.
Frankly, Israel has us by the balls. Less an ally and more a dominatrix.