I've only just recently started using mpaste, and I found the guide in the wiki to be a bit overly technical and difficult to understand without an intimate knowledge of the programming behind it.  So I figured I'd learn through trial and error and share what I discovered, its really quite simple but many people don't know how to use it.
This is one of the most useful tools you have in your arsenal if you can bend it to your will.

Mpaste is basically a copy mirrored tool.  It will paste your selection, only mirrored along the axis you determine.  What I mean by mirrored is the selection will be reversed, as if you held a mirror up to it.

You control Mpaste by calling on XYZ modifiers.  Lower case letters xyz represent non-selected axis, while uppercase letters XYZ represent the axis your selection will mirror on.

Examples,
I have a pillar,

 
I want anouther one identical to it, right next to it, only flipped around.
The command I will use is "/mpaste pillar Xyz".  Notice only the X is in capitals, so only the X axis will be flipped.

Now lets say I want the same pillar, but flipped on it's head.  I'll type "/mpaste pillar xyZ".  Only the Z is capitalized this time, so the selection will only flip along the Z axis.

Ok now lets get a bit more fancy, say I want this spectacular pillar flipped around AND turned upsidedown.  I type "/mpaste pillar XyZ".  In this case the X and the Z axis are capitalized, so BOTH of those axis are selected.  Lets see what we get.

So how can this be used in a real building situation?  Well here is an image of my latest project, as you can see, one side is more complete than the other.

So to apply that to the other side, I just need to use a bit of creative selecting, and pasting.  Once I've made my selection of the more complete half, I use "/mpaste body Xyz".  To flip it along the X axis, and we're all done.

Hope this helps, happy building everyone.