Minimal at first, because as others have noted, the US essentially had no ability to enforce it until the late 19th century. It worked because it was also in Britain's interest. The main effect is that the Monroe Doctrine formed the basis of the "Roosevelt Corollary" from the turn of the century into the 1970s. The Monroe Doctrine, so modified, gave some quasi-legitimacy to US intervention in central america whenever local regimes either became unstable, were in risk of economic default, or leaned diplomatically toward potentially enemy European powers. Perhaps, without the Monroe Doctrine that in effect created the notion that the new world was a US protectorate, the US habit of intervening in latin america would never have occurred.