I just had to model a foot cutout in the bottom of one of my bodies and since the foot is symmetrically identical in all four corners, it's the perfect candidate for the mirror function. However, since the mirror function only lets you specify one plane, it would require two mirror functions... one to mirror on the horizontal, then selecting both the original feature and it's mirrored sibling, adding a second mirror to the vertical.
If you could specify more than one plane, this could have been achieved in a single step.
Also, this may be a bug, but when I tried the second mirror operation (mirroring the oriiginal along with the first mirror) only the original was mirrored this second time even though it showed I had two items selected... the original item and the original mirror. I think that's a bug. I ended up fighting with it long enough to throw in the towel and just draw everything out by hand with no mirrors. Not cool, but I got my work done.