To read more about eyepiece projection for macro look at this page.
You can use this calculator to find out the magnification and focus distance given the specifications of your lens and the length of extension.
The drawbacks with using coupled reversed lenses is that you are adding more optics so any flaws in the lens get added. The drawbacks with using extension tubes or bellows is you lose automation, some speed of the lens. They are clumsy and bellows are fragile. You also need to focus, meter and set the aperture manually unless you pay a lot more for electronics/automation. There is a gizmo you can buy that allows you to adjust the aperture on Canon lenses be they reversed or mounted on "dumb" tubes/bellows for ~$200 though. There are ways to set the aperture on some cameras like Canon but it involves mount and dismounting the lens. And your viewfinder will go dim making it difficult to focus.
Here is a good overview of macro techniques, while geared for the Pentax user it is still applicable to all.