I've done this. It sucks. If you don't have a focusing rail on your tripod, its impossible to use. Depth of field is about .5mm if you stop as far down as you can go. I have a shot of a penny that was parallel to the sensor where Lincoln's eyes are sharp, but his nose isn't.

Frankly, reversing a lens on the front of another is only useful if you need extreme amounts of magnification and don't care as much about picture quality. On the plus though, it is really cheap to do. Hell, probably cheaper than buying a can a pringles and those caps that we all just have "laying around".

I much prefer using proper extension tubes or a true macro lens. Just because you made something small look big doesn't mean it looks good too.