The first lock that you put on your bike, regardless how cheesy, is about 90% effective because it requires some kind of tool to defeat. As you progress from 90% to 99% you engage in a progressively higher tech and progressively higher cost game of one-up-man-ship with the thief. Ultimately you will always lose because there is no 100% effective lock.

If you are securing your bike in a high theft risk area, like any college campus, my advice is to ride a POS bike and try to park it next to nicer bikes.