The Boolean2Objects command in V5 and V4 lets you select two objects and cycle through the four possible outcomes of a Boolean operation (with preview):

• Unordered List ItemUnion (fusing the two together)
• Subtract A from B
• Subtract B from A
• Intersection (leaving only the overlapping volume of A and B)

This is especially helpful if you have open objects, as it is not necessary to look at surface directions. One of the four possibilities will always be what you want.